feat: Add Muon optimizer for FSDP training

[nguyenhoangthuan99]

• Add verl/optim/muon.py with Muon and MuonWithAdamW optimizer classes
• Add verl/optim/init.py to export the optimizer classes
• Add verl/trainer/config/optim/muon.yaml for Hydra config
• Add examples/muon/config/ppo_trainer_muon.yaml example training config
• Update verl/workers/config/optimizer.py to add MuonOptimizerConfig and handle Muon in build_optimizer()

Muon (MomentUm Orthogonalized by Newton-Schulz) is designed for 2D matrix parameters in neural networks. It should be combined with AdamW for non-matrix parameters (embeddings, biases, gains, etc.).

Reference: https://github.com/KellerJordan/Muon

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