Enhancement: Support zstd compression levels `fastest` and `better`

Restic now supports the zstd compression modes `fastest` and `better`. Set the
environment variable `RESTIC_COMPRESSION` to `fastest` or `better`, or pass the
same values with the `--compression` option.

https://github.com/restic/restic/issues/4728
https://github.com/restic/restic/pull/5321
