Expose MockUserCredential

[thedalelakes]

Currently, if I want to mock the method User.linkWithProvider, which returns a UserCredential, I have to implement my own mock for UserCredential.

I'd prefer to simply re-use your implementation so I can have something like the following:

when(() => firebaseAuth.currentUser!.linkWithProvider(any()))
              .thenAnswer((_) async => Future.value(MockUserCredential(
                        mockUser: firebaseAuth.currentUser!,
              ));

Any reason why MockUserCredential shouldn't be exported and made available?

[atn832]

Despite the name, firebase_auth_mocks tries to fake as much as possible as you did so well in #91 (thanks btw!), so we don't want to use Mockito's pattern of mocking custom values.

Btw we don't export MockUserCredential and other custom implementations of the Firestore API because unit tests should use the public Firestore API as much as possible.