Xet download workflow

[hanouticelina]

🏗️ This PR is a work in progress, don't review yet.

Partially resolves #2713.

This PR adds the Xet download workflow implemented in xetpoc_huggingface_hub (internal).Tthe upload one will be integrated in a separate PR

The main branch for xet storage integration into huggingface_hub is xet-integration.

Main changes:

• Make hf_xet available as an optional dependency via pip install huggingface_hub[hf_xet]
  Note: since it's a common part for download and upload, this has been pushed directly into xet-integration branch.
• Integrate changes from the xet poc for the download workflow only.
• Add tests.
• Add documentation.

to try it in from this branch:

pip install -e ".[dev,hf_xet]"
export HF_DEBUG=1 #  if you want to set huggingface_hub logger to debug level
huggingface-cli download huggingface/distilbert-base-uncased-xet

cc @bpronan @assafvayner @rajatarya

[HuggingFaceDocBuilderDev]

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