chore: Update default API version to current date dynamically

The commit dynamically updates the `defaultApiVersion` in Sanity's client configuration, calculated based on the current date. This modification is intended to ensure that the most relevant data is always fetched. Changes were also made to handle the case where no token is provided, removing the need for a hard assertion and setting the headers correspondingly.

packages/sanity/sanity_client/lib/client.dart

@@ -15,7 +15,16 @@ final class SanityConfig {
   final Perspective perspective;
   final bool explainQuery;
 
-  static const defaultApiVersion = 'v2021-10-21';
+  static final String defaultApiVersion = (() {
+    final today = DateTime.now();
+    final parts = [
+      today.year.toString(),
+      today.month.toString().padLeft(2, '0'),
+      today.day.toString().padLeft(2, '0')
+    ].join('-');
+
+    return 'v$parts';
+  })();
 
   SanityConfig({
     required this.projectId,
@@ -29,7 +38,8 @@ final class SanityConfig {
         this.apiVersion = apiVersion ?? defaultApiVersion,
         this.perspective = perspective ?? Perspective.raw,
         this.explainQuery = explainQuery ?? false {
-    assert(this.token.trim().isNotEmpty, 'Invalid Token provided');
+    assert(this.token.trim().isNotEmpty,
+        'Invalid Token provided. Setup an API token, with Viewer access, in the Sanity Management Console.');
   }
 }
 
@@ -46,7 +56,9 @@ class SanityClient {
     final UrlBuilder? urlBuilder,
   })  : httpClient = httpClient ?? http.Client(),
         urlBuilder = urlBuilder ?? SanityUrlBuilder(config),
-        _requestHeaders = {'Authorization': 'Bearer ${config.token}'};
+        _requestHeaders = config.token != null
+            ? {'Authorization': 'Bearer ${config.token}'}
+            : {};
 
   Future<SanityQueryResponse> fetch(Uri uri) async {
     final response = await httpClient.get(uri, headers: _requestHeaders);