build: add support for zstd image compression

[milas]

Changes

A pretty mechanical plumbing of a new --compression flag through the various machinery.

The default is still gzip for backwards compatibility, but --compression=zstd can be specified as an alternative.

Container runtimes have good zstd support these days and it compresses better/with less CPU than gzip.

Context

Starting with google/go-containerregistry v0.21.8, it's possible to make images with correctly named ZSTD-compressed layers.

(Previously, it was possible to write zstd-compressed layer data, but it'd still be .tar.gz, which was confusing at best but more likely to break downstream tooling.)

See google/go-containerregistry#2382

The go.mod here already picked that up, so no dependency update is necessary.

[milas]

FYI I did a quick scan of other open issues/PRs -- I do see there's WIP for adding EROFS (very cool!) @ #2249, which I don't think really conflicts. Arguably, this can work together with that since EROFS layers can similarly be gzip/zstd/etc compressed.