Add support for file write/upload operations with HfHubRepository

[shadeMe]

Description

This PR introduces support for uploading files to Hugging Face Hub repositories through the current RepositoryFile API. In the case of fsspec repositories, we support implicit streaming of data using buffered IO. However, this is not possible with HF repositories due its usage of Git as a backing store. So, we need to allocate and write to files on the host's local storage device before uploading them as a Git commit operation to the remote repo.

The current implementation hides this detail by using a proxy to represent the remote file that we want to write to. The HfHubFile class wraps either a locally cached file from a HF repo or a temporary file on the local storage. The user can open and write to the latter like they would with any (fsspec) file - its contents will be uploaded to the repo as soon as the file handle is closed.

Furthermore, we also introduce the concept of transactional file writes using a context manager. This lets the user batch multiple file operations that get uploaded to the repo as a single commit. The fsspec implementation of transactions will be implemented in a follow-up PR.

Other changes:

• Repository.file returns a lazily-loaded file, i.e., its existence is only checked when the file is opened using the open method (or when the exists method is called).
• Fixed a bug in FsspecRepository.open that called unstrip_protocol on a None object.

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.

[danieldk]

🎉

Added some comments

[rmitsch]

I'm not familar with the codebase yet, so my comments are somewhat superficial. Overall this looks reasonable to me. I'll do a second pass after the first batch of comments has been addressed.

[rmitsch]

🎉