[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.

Start by locating the iOS GameCenterAuthProvider implementation and its GetCredential entry point; compare the setAuthenticateHandler UIViewController callback with the macOS NSViewController form. Done means the provider can exchange macOS Game Center authentication for a Firebase credential and support the shared user flow.

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