feat: Add System.Text.Json serialization/deserialization supports

[filzrev]

This draft PR intended to add functionality for gradual migration from Newtonsoft.Json to System.Text.Json. (Related issue #9727)

1. Commit (b3c5282)

• Refactor JsonUtility related code to support switching NewtonsoftJson/SystemTextJson by type.

2. Commit (f47673c)

• Add test infrastructure code to validate serialization/deserialization results.
• Validate JSON serialization results has no diffs between Newtonsoft.Json/SystemTextJson.
• Validate JSON serialize/deserialize roundtrip result object comparison between Newtonsoft.Json/SystemTextJson.
• Validate YAML related roundtrip results. (Object->YAML->Object and Object->JSON->Object)

3. Commit (165bb8a)

• Switch XRefMap serialization/deserialization to use JsonUtility and System.Text.Json.
• Add XRefMap serialization/deserialization tests.

4. Commit (3b3bd35)

• Add JSON serialization/deserialization round trip tests with JsonUtility

5. Commit (aece12e)

• Add System.Text.Json based serialization/deserialization for MarkdigExtensionSetting.

6. Commit (f636b8d)

• Add System.Text.Json based serialization/deserialization for ManagedReference.PageViewModel
  • Modify CompositeDictionary related property to add private init and JsonInclude attribute
    (It's required because System.Text.Json can't deserialize getter-only property)

TODO:

• Add JsonConverter for System.Text.Json
• Verify serialization/deserialization compatibility by unit tests.
• Switch to use System.Text.Json based serialization/deserialization for other types.