[firebase_auth] IdTokenResult object needs to have a token constructor

[mehmetf]

We have applications that implement FirebaseUser class. In order to support this use case, classes such as IdTokenResult need to be open to being constructed as opposed to having only an internal API.

I propose to add:

  IdTokenResult.fromToken(String token):
      _data = <String, String>{'token': token},
      _app = null;

to it.

[mehmetf]

I will take this issue on @collinjackson . It would also be great if you could give me super powers in this repo so I can assign issues, merge PRs etc.

[mehmetf]

The decision is to implement all classes instead of returning firebase_auth owned ones.