snapshot_download should offer a way to get progress

[jeffmaury]

When processing a model download, it should be fine to get progress about the download.

Proposal: offer a callback that returns the percentage, maybe after each file is downloaded ?

WDYT @axel7083

[coyotte508]

For reference, there are two function to upload files, uploadFiles which returns a Promise, and uploadFilesWithProgress which returns an async generator.

The same can be done here