Implement a `from_pephub` method

[nleroy917]

This should be done in the core crate and will be useful for grabbing and instantiating PEPs from the PEPhub API. We can expose this method to Python.

[nleroy917]

Useful inspo: https://github.com/huggingface/hf-hub

Very minimal and simple

[nleroy917]

I'd argue for this to be its own crate in the workspace. Name ideas?

• pephub-client
• pephub-api
• pephub-cache
• pepstore
• pepfetch

[nleroy917]

This is half-baked. Lots to imrpove I think so will keep it here for now

[khoroshevskyi]

I think function from pephub doesn't work right now