Add zstd support as a compression method

## Summary

Add `zstd` codec support.

## Desired Behaviour

CouchDB already supports multiple compression codecs (by the way, which exactly? Cannot find the documentation about it), so we want just to have the option to use `zstd` as another compression codec.

## Additional context

We want to use `zstd` since it seems like one of the most advanced compression on the market and has a very good CPU/compression ratio.

No files or tests are named. Start by locating CouchDB’s existing compression-codec entry points and documenting which codecs are currently supported, then trace how codec selection is configured and tested. Done means zstd can be selected as a compression codec and the relevant tests cover it.

Written by the indexing model from the issue text.