Provide progress callback support for downloading index and packs

[flit]

The Python API needs to support a caller-provided progress callback for the cache_descriptors() and download_pack_list() methods.

Ideally the progress callback would be provided with

• The name of the file currently being downloaded.
• URL of current file.
• Percent complete for current file.
• Maybe also a total percent complete.

[theotherjimmy]

@flit I would like to implement this, and I should have all of the information needed through the Rust side of the API. It is in part a matter of wiring things up. Be aware that if you ask for more than one file to be downloaded, that they may be downloaded in parallel. With your proposed API, you would get calls to that progress callback with different file, url pairs interleaved. This is probably fine, I just want to make sure we're all on the same page.