[FR] Missing claims on GetToken

### Feature proposal

* Firebase Component: Auth (Auth, Core, Database, Firestore, Messaging, Storage, etc)

On other platforms, we have a way of accessing the claims associated with a token. For instance on Android: https://firebase.google.com/docs/reference/android/com/google/firebase/auth/GetTokenResult

On Desktop C++ the `GetToken` only returns a string: https://firebase.google.com/docs/reference/cpp/class/firebase/auth/user#gettoken

We should support claims.

Requested on FlutterFire repo: https://github.com/firebase/flutterfire/issues/11949

从 Desktop C++ Auth User::GetToken API 开始，并将其仅返回字符串的结果与 issue 中链接的 Android GetTokenResult 参考进行比较。审查相关的 FlutterFire 请求，并确定 claims 的表示方式、API 形态、兼容性要求以及使该功能被视为完成所需的验证。

由索引模型根据 Issue 内容生成。