feat(publish): include blake2b hash in upload form

[woodruffw]

Summary

This adds the blake2_256_digest form item on uploads, since some third-party indices apparently expect it. This is slightly different from the (rejected) idea of including an md5_digest since Blake2b is not broken like MD5 is.

To do this, I've tweaked the hash_file helper to return multiple hashes instead of just one. This makes getting the hashes-by-type out of the function slightly less pretty in terms of what Rust can assert about safe indexing.

Closes #15781.

Test Plan

This should be covered under existing upload tests, since we include the blake2 hash unconditionally.

[woodruffw]

Flagging: I thought iter + find here would be slightly cleaner than indexing with [0] and [1], since we may not want to assume the return order is the same as the request order in the hash_file parameters.

[konstin]

We can assume the order, but this is fine too