Implement zstd, gzip, bzip2 (de)compression as alternative to xz decompression
So far gzip, zstd, and bzip2 compressed images would need to be decompressed manually before "restoring"/writing the image to a disk/partition. Only xz decompression so far is supported.
It would be nice to support more formats as alternative to xz because of the current security/trust issues. So far xz was anyway a rather bad choice for image compression because it's usually very slow.
We could also decide to drop all decompression support but for larger images with a lot of empty space we the manual decompression sometimes doesn't result in sparse files but zero-filled files that take a lot of disk space, or more than available. On the fly decompression makes it easier for users to write out downloaded images.
Related: We could also support creating compressed images, as with something like gzip or zstd the compression overhead can be small. As far as I recall the images we create are sparse but as soon as users would copy or upload them they could be bloated up again, which would be avoided with a layer of compression.