Export AuthProvider so that it can be used in firebase_auth_mocks

[gaburielcasado]

Putting this in General as I think it doesn't fit into features.

I ran across an issue related to unit testing. I'm currently using firebase_auth_mocks in unit tests, but one of the FirebaseAuth methods is not supported: signInWithPopup.

I was ready to write an implementation and open a pull request, however AuthProvider is not exported, so I cannot override the method. My question is simple, would it be a problem if we added an export declaration for AuthProvider? I'm ready to open a pull request :)

Edit: I now know that I can import the AuthProvider class like this: import 'package:firebase_auth_platform_interface/src/auth_provider.dart';

Would that be acceptable?