Add transport compression to attic push

[DarkKirb]

My buildserver is behind a 4Mbps uplink, so it is very beneficial for me if attic supported transport compression. This pull requests adds transport compression to both the server and the client.

It adds support for the Content-Encoding header to /api/v1/upload-path, with the following compression formats:

• Deflate (standard)
• Gzip (standard)
• Brotli (standard)
• xz (supported by attic already)
• zstd (supported by attic already)

As per the standard, it compresses the whole request body, including the narinfo file. X-Attic-Nar-Info-Preamble-Size refers to the byte offset in the decompressed body. Ideally this would reuse the request decompression middleware from tower-http, but i didn’t figure out how to get it to work, due to differing body types.

The client portion of the change adds a configuration option that changes transport compression to the server. This option is scoped to each server, as for example an attic server running on LAN might not need transport compression.

The client compression code is a bit suboptimal, due to how async_compression and reqwest use different types, and more efficient ways of adapting an AsyncRead to a Stream require tokio (which fwict would make it more difficult to adapt a stream into an AsyncRead first)

[DarkKirb]

I have deployed it in prod and with default settings it seems to give me a 2x performance improvement in most larger derivations (1MB/s instead of 500kB/s) and no change in derivations with large amounts of random data (tarballs for example). I think i can still improve compression ratios and therefore speed

[gador]

This looks like a really cool feature. Can you rebase it to the current main branch?