Release v2.0.0

• Complete refactor and codebase cleanup for better maintainability and performance.
• Updated the Flutter package dependency firebase_auth to version ^5.2.0 for improved compatibility and security updates.
• Revised and cleaned up the README to reflect the latest changes and provide clearer setup instructions.
• Upgraded Android implementation to use com.google.android.gms:play-services-games-v2+ for enhanced support of Play Games Services v2.