[FR]: Reuse GameCenterAuthProvider implementation on macOS

[solodon4]

Feature proposal

• Firebase Component: Auth

GameKit is supported by Apple on both iOS and macOS and its Game Center authentication API is 99% reusable between OSes, with the only exception being UIViewController vs NSViewController parameter in GKLocalPlayer:setAuthenticateHandler. 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 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.

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