Add commit_mode immediate/queued/flush to create_commit

[Wauplin]

Add a new parameter commit_mode to HfApi.create_commit with value:

• "immediate": commit is processed immediately (default)
• "queued": commit is pending. No commit info returned.
• "flush": all pending commits are processed and merged into one commit

By default, no commit mode is sent to the server which default to immediate commit.

Goal is to always upload from multiple nodes without getting concurrency or rate limit issues.

Related PRs:

• backend https://github.com/huggingface-internal/moon-landing/pull/15836 (internal repo)
• JS SDK add queued commit support huggingface.js#1849

TODO:

• test on staging Hub
• return payload in case of queued commit
• add commit_mode to upload_file / upload_folder / delete_file methods once syntax is definitive
• add tests

cc @coyotte508

[HuggingFaceDocBuilderDev]

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