[FR]: Reuse GameCenterAuthProvider implementation on macOS

### Feature proposal

* Firebase Component: Auth

[GameKit](https://developer.apple.com/documentation/gamekit?language=objc) is supported by Apple on both iOS and macOS and its [Game Center authentication API](https://developer.apple.com/documentation/gamekit/authenticating_a_player?language=objc) is 99% reusable between OSes, with the only exception being `UIViewController` vs `NSViewController` parameter in [`GKLocalPlayer:setAuthenticateHandler`](https://developer.apple.com/documentation/gamekit/gklocalplayer/1515399-authenticatehandler?language=objc). If you guys massage your iOS implementation of `GameCenterAuthProvider` to also work on macOS, which from my experience should be pretty straightforward, then we will be able to use `GameCenterAuthProvider::GetCredential` to exchange it for a Firebase credential and thus have a unified user support across entire apple eco-system. This is particularly interesting to developers offering [universal purchase](https://developer.apple.com/support/universal-purchase/) because Game Center user IDs are shared between iOS and macOS thus enabling us to recognize the same user in Firebase across those operating systems.