Support proxy and user-agent in fsspec calls
Since we switched to the new HfFileSystem we no longer apply user's proxy and user-agent.

Using the HTTP_PROXY and HTTPS_PROXY environment variables works though since we use aiohttp to call the HF Hub.

This can be implemented in `_prepare_single_hop_path_and_storage_options`.

Though ideally the `HfFileSystem` could support passing at least the proxies