feat(auth, apple): Game Center sign-in support

[russellwheatley]

Description

I've had to change the way we get the AuthCredential on Apple platforms. Game Center credential is different to the other providers. It requires a callback to receive. See link: https://firebase.google.com/docs/auth/ios/game-center#authenticate_with_firebase
Here is the snippet:

// Get Firebase credentials from the player's Game Center credentials
[FIRGameCenterAuthProvider getCredentialWithCompletion:^(FIRAuthCredential *credential,
                                                         NSError *error) {
  // The credential can be used to sign in, or re-auth, or link or unlink.
  if (error == nil) {
    [[FIRAuth auth] signInWithCredential:credential
                              completion:^(FIRUser *user, NSError *error) {
      // If error is nil, player is signed in.
    }];
  }
}];

Previously, we called a synchronous method to get the credential: https://github.com/firebase/flutterfire/blob/master/packages/firebase_auth/firebase_auth/ios/Classes/FLTFirebaseAuthPlugin.m#L641C33-L641C66

But I had to make this a callback to allow for Game Center credential retrieval: https://github.com/firebase/flutterfire/pull/12228/files#diff-926b26fe3f9119fb3548066f1b05028306ff8766f9a04abba0fefdb5dac5c0b2R642-R645

This method is used for signInWithCredential, linkWithCredential(), reauthenticateWithCredential() & updatePhoneNumber() API which will allow the user to call these methods signInWithCredential, linkWithCredential(), reauthenticateWithCredential() with Game Center credential.

Testing Game Center sign-in

1. Add Game Center capability on Xcode project which also brings in framework for Game Player.
2. Create an app in "App Store Connect" and select the "Game Center" option to allow authentication.
3. Implement the following in AppDelegate to login with Game Center:

__weak GKLocalPlayer *localPlayer = [GKLocalPlayer localPlayer];
localPlayer.authenticateHandler = ^(UIViewController *gcAuthViewController,
                                    NSError *error) {
  if (gcAuthViewController != nil) {
    // Pause any activities that require user interaction, then present the
    // gcAuthViewController to the player.
  } else if (localPlayer.isAuthenticated) {
    // Player is signed in to Game Center. Can now login with Game Center credential
	NSLog(@"logged in with Game Center");
  } else {
    // Error
  }
};

4. Now able to login with Game Center with Firebase.

Tested signInWithCredential(), linkWithCredential() & reauthenticateWithCredential()

Related Issues

closes #12191
closes #9441

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Breaking Change

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[Lyokone]

I don't see any Pigeon related changes on the Dart part, is there some files missing?

[russellwheatley]

This method is internal to the plugin and isn't part of Pigeon API.

[Lyokone]

Oh nice didn't realise thanks :)