FR: Support zstd compression

[ZacSweers]

This is what github actions/cache and I believe circleCI use for cache artifact compression, as it's much faster than zip and tar gzip.

[toote]

I had never heard of zstd but the compression implementation is something I wanted to change anyway. I'm reviewing a better way to allow for extensions to the supported compression programs or options so that this kind of needs don't require changing the plugin itself (as there will be no end to them).

Current idea is to do something similar to how backends of the plugin work: allow specifying whatever you want as compression as long as a single script exists to handle the passthrough of options to the compression/uncompression programs. What do you think about it? Any ideas or needs outside just supporting a different executable?

[ZacSweers]

That makes sense to me (re: bringing other compressions)