bottomless: add zstd compression option

[psarna]

Transplanted from libsql/sqld#780

[psarna]

xz compression emprically showed ~20-25x better compression ratio than gzip. In my tests xz proved to actually be faster too, because instead of having to write ~200MiB to disk as gzip would, it wrote ~10MiB, at the cost of more cpu used. We should test first, but then strongly consider making xz default.
To set it up, it's enough to set

LIBSQL_BOTTOMLESS_COMPRESSION=zstd

as an env var

[psarna]

@MarinPostma downgraded to draft, because I see some spontaneous errors when testing xz compression on large random data (one that does not compress well). I'll evalutate other compression implementations as well, because xz support looks a little abandoned in the async_compression crate

[psarna]

Rebranded to zstd. It's a modern one that also powers ScyllaDB, and it first and foremost passed all correctness tests on which xz and bzip2 failed. It's also way faster than any of the other ones, while still providing compression ratios of 12-15 where gzip gives mere 2.