[FEATURE] Implement self-play for two-player zero-sum games

[RPegoud]

Description: Add self-play versions of DQN and PPO for two-player zero-sum games in PGX environments.

Checklist:

• [ ] Determine how to keep the value estimation consistent (e.g. flip the board or reverse the discount for opponent values)
• [ ] Add PGX environment configs
• [ ] Implement self-play for DQN
• [ ] And for PPO
• [ ] (optional) If possible, for AlphaZero