Add zstandard support for wheels (#15645)

## Summary

This PR allows pyx to send down hashes for zstandard-compressed
tarballs. If the hash is present, then the file is assumed to be present
at `${wheel_url}.tar.zst`, similar in design to PEP 658
`${wheel_metadata}.metadata` files. The intent here is that the index
must include the wheel (to support all clients and support
random-access), but can optionally include a zstandard-compressed
version alongside it.

crates/uv-client/src/flat_index.rs

@@ -305,6 +305,7 @@ impl<'a> FlatIndexClient<'a> {
                 upload_time_utc_ms: None,
                 url: FileLocation::AbsoluteUrl(UrlString::from(url)),
                 yanked: None,
+                zstd: None,
             };
 
             let Some(filename) = DistFilename::try_from_normalized_filename(filename) else {