Add Multi-Storage Client (MSC) support for checkpointing

[shunjiad]

Megatron-LM supports saving and loading checkpoints from the object storage via MSC. Megatron-Bridge introduces three new files to the checkpoint directory, so MSC must support those files.

• latest_train_state.pt
• iter_{iteration}/run_config.yaml
• iter_{iteration}/train_state.pt

To save the checkpoints to the object storage, the user can specify the MSC URL in the checkpoint section.

checkpoint:
  # Save checkpoints to S3 via MSC
  save: msc://s3-profile/checkpoints
  load: msc://s3-profile/checkpoints

Closes #229

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[ananthsub]

/ok to test 9bcf8f4