Replace native schemaToXmlString (#217)

- Add SchemaTestUtils class that with schemaToXmlString method
- To convert schema to an xml string I added @itwin/ecschema-locaters as they have helper to do this
- In an effort to replace internal apis that are currently deprecated and will be removed in itwin js 5.0 release, we will replace `nativeDb.schemaToXmlString` with `SchemaTestUtils.schemaToXmlString`

change/@itwin-imodel-transformer-9ed49a07-59ed-4232-99e0-f7b895aa8d4e.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "add alternative schemaToXmlString",
+  "packageName": "@itwin/imodel-transformer",
+  "email": "Daniel.Erbynn@bentley.com",
+  "dependentChangeType": "patch"
+}