Add an object generator.

I guess I never PRed this...

Adds ability to override built-in objects to custom ones so you can do things like add shadows:
```
RiveAnimation.asset(
  'assets/shadowtext.riv',
  objectGenerator: (coreTypeKey) {
    switch (coreTypeKey) {
      case ShapeBase.typeKey:
        return ShadowRiveShape();
    }
    return null;
  },
```

![image](https://github.com/rive-app/rive/assets/454182/cae261b1-8005-4c49-9714-3bb1dca50cde)

```
class ShadowRiveShape extends Shape {
  Paint? _shadowPaint;
  @OverRide
  void onAdded() {
    if (name == 'ShadowShape') {
      // This was named "ShadowShape" we use that as a simple way to determine
      // that we want to add a drop shadow at runtime.
      _shadowPaint = Paint()
        ..imageFilter = ImageFilter.blur(
          // Make it a 10x10 black blur
          sigmaX: 10,
          sigmaY: 10,
          tileMode: TileMode.clamp,
        );
    }
    super.onAdded();
  }

  @OverRide
  K? clone<K extends Core<CoreContext>>() {
    var shape = ShadowRiveShape();
    shape.copy(this);
    return shape as K;
  }

  @OverRide
  void draw(Canvas canvas) {
    var shadow = _shadowPaint;
    if (shadow != null) {
      // We draw a blurred version of this shape before regular drawing. Make
      // sure to respect clipping rules for the shadow too.
      bool clipped = clip(canvas);
      var path = pathComposer.fillPath;
      canvas.save();
      if (!fillInWorld) {
        canvas.transform(worldTransform.mat4);
      }
      // Offset the shadow to the bottom right (change to whatever you want).
      canvas.translate(10, 10);
      canvas.drawPath(path, shadow);

      canvas.restore();

      if (clipped) {
        canvas.restore();
      }
    }

    // Draw the regular shape.
    super.draw(canvas);
  }
}
```

Diffs=
99a2215da Add an object generator. (#6805)
7cb7eb812 Upgrade rive_wasm to the new premake system (#6789)

Co-authored-by: Luigi Rosso <luigi-rosso@users.noreply.github.com>

.rive_head

@@ -1 +1 @@
-74e649ee25a9d65fd428f32d3dbd144e54c38aa8
+99a2215daeffdfe537e2a900113a9d59ee7b9d3f

lib/src/rive_file.dart

@@ -40,17 +40,21 @@ import 'package:rive_common/rive_text.dart';
 
 import 'package:rive_common/utilities.dart';
 
-Core<CoreContext>? _readRuntimeObject(
-    BinaryReader reader, HashMap<int, CoreFieldType> propertyToField) {
+typedef Core<CoreContext>? ObjectGenerator(int coreTypeKey);
+
+Core<CoreContext>? _readRuntimeObject(BinaryReader reader,
+    HashMap<int, CoreFieldType> propertyToField, ObjectGenerator? generator) {
   int coreObjectKey = reader.readVarUint();
-  Core<CoreContext>? instance;
-  switch (coreObjectKey) {
-    case ArtboardBase.typeKey:
-      instance = RuntimeArtboard();
-      break;
-    case NestedArtboardBase.typeKey:
-      instance = RuntimeNestedArtboard();
-      break;
+  Core<CoreContext>? instance = generator?.call(coreObjectKey);
+  if (instance == null) {
+    switch (coreObjectKey) {
+      case ArtboardBase.typeKey:
+        instance = RuntimeArtboard();
+        break;
+      case NestedArtboardBase.typeKey:
+        instance = RuntimeNestedArtboard();
+        break;
+    }
   }
   var object = instance ?? RiveCoreContext.makeCoreInstance(coreObjectKey);
 
@@ -153,6 +157,7 @@ class RiveFile {
   RiveFile._(
     BinaryReader reader,
     this.header,
+    ObjectGenerator? generator,
     this._assetLoader,
   ) {
     /// Property fields table of contents
@@ -162,7 +167,7 @@ class RiveFile {
     var artboardLookup = HashMap<int, Artboard>();
     var importStack = ImportStack();
     while (!reader.isEOF) {
-      final object = _readRuntimeObject(reader, propertyToField);
+      final object = _readRuntimeObject(reader, propertyToField, generator);
       if (object == null) {
         // See if there's an artboard on the stack, need to track the null
         // object as it'll still hold an id.
@@ -320,18 +325,23 @@ class RiveFile {
   /// Loading assets documentation: https://help.rive.app/runtimes/loading-assets
   /// {@endtemplate}
   ///
+  /// Provide an [objectGenerator] if you want to override any built-in Rive
+  /// types.
+  ///
   /// Will throw [RiveFormatErrorException] if data is malformed. Will throw
   /// [RiveUnsupportedVersionException] if the version is not supported.
   factory RiveFile.import(
     ByteData bytes, {
     @Deprecated('Use `assetLoader` instead.') FileAssetResolver? assetResolver,
     FileAssetLoader? assetLoader,
+    ObjectGenerator? objectGenerator,
     bool loadCdnAssets = true,
   }) {
     var reader = BinaryReader(bytes);
     return RiveFile._(
       reader,
       RuntimeHeader.read(reader),
+      objectGenerator,
       FallbackAssetLoader(
         [
           if (assetLoader != null) assetLoader,
@@ -355,6 +365,7 @@ class RiveFile {
     ByteData bytes, {
     FileAssetLoader? assetLoader,
     bool loadCdnAssets = true,
+    ObjectGenerator? objectGenerator,
   }) async {
     if (!_initializedText) {
       /// If the file looks like needs the text runtime, let's load it.
@@ -367,6 +378,7 @@ class RiveFile {
       bytes,
       assetLoader: assetLoader,
       loadCdnAssets: loadCdnAssets,
+      objectGenerator: objectGenerator,
     );
   }
 
@@ -384,6 +396,7 @@ class RiveFile {
     AssetBundle? bundle,
     FileAssetLoader? assetLoader,
     bool loadCdnAssets = true,
+    ObjectGenerator? objectGenerator,
   }) async {
     final bytes = await (bundle ?? rootBundle).load(
       bundleKey,
@@ -393,6 +406,7 @@ class RiveFile {
       bytes,
       assetLoader: assetLoader,
       loadCdnAssets: loadCdnAssets,
+      objectGenerator: objectGenerator,
     );
   }
 
@@ -410,13 +424,15 @@ class RiveFile {
     @Deprecated('Use `assetLoader` instead.') FileAssetResolver? assetResolver,
     FileAssetLoader? assetLoader,
     bool loadCdnAssets = true,
+    ObjectGenerator? objectGenerator,
   }) async {
     final res = await http.get(Uri.parse(url), headers: headers);
     final bytes = ByteData.view(res.bodyBytes.buffer);
     return _initTextAndImport(
       bytes,
       assetLoader: assetLoader,
       loadCdnAssets: loadCdnAssets,
+      objectGenerator: objectGenerator,
     );
   }
 
@@ -430,12 +446,14 @@ class RiveFile {
     String path, {
     FileAssetLoader? assetLoader,
     bool loadCdnAssets = true,
+    ObjectGenerator? objectGenerator,
   }) async {
     final bytes = await localFileBytes(path);
     return _initTextAndImport(
       ByteData.view(bytes!.buffer),
       assetLoader: assetLoader,
       loadCdnAssets: loadCdnAssets,
+      objectGenerator: objectGenerator,
     );
   }
 

lib/src/widgets/rive_animation.dart

@@ -66,6 +66,11 @@ class RiveAnimation extends StatefulWidget {
   /// {@macro Rive.behavior}
   final RiveHitTestBehavior behavior;
 
+  /// Rive object generator to override built-in types and methods to, for
+  /// example, interject custom rendering functionality interleaved with Rive
+  /// rendering.
+  final ObjectGenerator? objectGenerator;
+
   /// Creates a new [RiveAnimation] from an asset bundle.
   ///
   /// *Example:*
@@ -86,6 +91,7 @@ class RiveAnimation extends StatefulWidget {
     this.controllers = const [],
     this.onInit,
     this.behavior = RiveHitTestBehavior.opaque,
+    this.objectGenerator,
     Key? key,
   })  : name = asset,
         file = null,
@@ -114,6 +120,7 @@ class RiveAnimation extends StatefulWidget {
     this.onInit,
     this.headers,
     this.behavior = RiveHitTestBehavior.opaque,
+    this.objectGenerator,
     Key? key,
   })  : name = url,
         file = null,
@@ -140,6 +147,7 @@ class RiveAnimation extends StatefulWidget {
     this.controllers = const [],
     this.onInit,
     this.behavior = RiveHitTestBehavior.opaque,
+    this.objectGenerator,
     Key? key,
   })  : name = path,
         file = null,
@@ -172,6 +180,7 @@ class RiveAnimation extends StatefulWidget {
     this.behavior = RiveHitTestBehavior.opaque,
   })  : name = null,
         headers = null,
+        objectGenerator = null,
         src = _Source.direct,
         super(key: key);
 
@@ -209,15 +218,18 @@ class RiveAnimationState extends State<RiveAnimation> {
       case _Source.asset:
         return RiveFile.asset(
           widget.name!,
+          objectGenerator: widget.objectGenerator,
         );
       case _Source.network:
         return RiveFile.network(
           widget.name!,
           headers: widget.headers,
+          objectGenerator: widget.objectGenerator,
         );
       case _Source.file:
         return RiveFile.file(
           widget.name!,
+          objectGenerator: widget.objectGenerator,
         );
       case _Source.direct:
         return Future.value(