Added Game Center Authentication Support

[imperiumplay]

Summary

Adds client-side Game Center authentication support to the Colyseus Unity SDK, enabling iOS developers to authenticate users through Apple's Game Center service.

Changes

• Added GameCenterCredentials class with required fields for Game Center authentication
• Added SignInWithGameCenter<T>() method for typed authentication responses
• Added SignInWithGameCenter() method for generic authentication responses

Integrates with existing auth flow, supports token caching, change callbacks, and error handling

Usage
var credentials = new GameCenterCredentials
{
    playerId = "G:123456789",
    bundleId = "com.yourcompany.yourgame",
    timestamp = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
    salt = "base64EncodedSalt",
    signature = "base64EncodedSignature",
    publicKeyUrl = "https://static.gc.apple.com/public-key/..."
};

var authData = await client.Auth.SignInWithGameCenter<User>(credentials);

Server Requirements

colyseus/colyseus#885