[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