Support compression algorithms other than zstd

[lengau]

We should be able to support compression algorithms other than zstd.

This is low priority because currently every system we need to support does this fine, but it would be nice for future systems or if specific packages want a different compression algorithm.

See:
#43 (comment)

[lengau]

This is a non-trivial change that will require some design work for debcraft.yaml:

1. The default compression algorithm for all debs can be the same as how snapcraft works.
2. We need to be able to have separate compression algorithms for the control tarball and data tarball. (Perhaps a dictionary of {control: <alg>, data: <alg>} is sufficient)
3. Individual packages also need to be able to override this.