🐛 [FIREBASE_AUTH] userAccessGroup setting signs users out, and there's no way to migrate existing users

[hamishjohnson]

What feature would you like to see?

I set the userAccessGroup in my app, however, it causes existing users to log out.

In the firebase swift docs, there's instructions on how to update the user using swift SDK getStoredUser and updateCurrentUser https://firebase.google.com/docs/auth/ios/single-sign-on

However, this capability is not included in the Flutter SDK, so I cannot perform the migration. If I deploy my app, all my users are going to get logged out, which is an especially big problem for anonymous users.