[Process] Need to add public documentation about how to sign-in using GameCenter

### [REQUIRED] Please fill in the following fields:

* Unity editor version: 2020.1.17f1
* Firebase Unity SDK version: 7.0.2
* Source you installed the SDK: .unitypackage (.unitypackage or Unity Package Manager)
* Problematic Firebase Component: Auth (Auth, Database, etc.)
* Other Firebase Components in use: Firestore (Auth, Database, etc.)
* Additional SDKs you are using: _____ (Facebook, AdMob, etc.)
* Platform you are using the Unity editor on: Mac (Mac, Windows, or Linux)
* Platform you are targeting: iOS, Android (iOS, Android, and/or desktop)
* Scripting Runtime:Mono (Mono, and/or IL2CPP)

### [REQUIRED] Please describe the question here:

Hi,

I would like to add the GameCenter Authentication with Firebase to my Unity Game (for iOS). But in the documentation there isn't the GameCenter part (only Play Games one). Is there a way to integrate Game Center auth with unity ?