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.
