Expanded descriptor compression support

We supported plaintext and gzip when downloading descriptors, but recently tor
added lzma and zstd support as well...

  https://gitweb.torproject.org/torspec.git/commit/?id=1cb56af

In practice I'm having difficulty finding an example of this working in the
live tor network...

  https://trac.torproject.org/projects/tor/ticket/25667

That said, gonna go ahead and push this since even setting lzma/zstd aside it's
a nice step forward since it allows callers to specify compression headers.