[Feature]: Game Center Authentication Support

Context & Description

Need Game Center authentication support in the auth module to allow players to sign in with their Apple Game Center accounts.

Use cases

• Standard authentication method for iOS games
• Reduces implementation effort for iOS developers using Colyseus
• Provides verified player identities through Apple's infrastructure
• Game center can be used for leaderboard, matchmaking, achievements etc

Proposed API

// Server-side
import { auth } from "@colyseus/auth";

auth.gamecenter.setup({
  bundleId: "com.example.game",
});

// Client sends Game Center credentials (signature, publicKeyURL, etc.)
// Server verifies signature against publicKeyURL (cached)
// Returns JWT token