crane: Support copy over all non-digest tags

[stevehipwell]

I need to sync a complex set of tags where the non-digest tags are mutable, currently I'm doing this by copying each non-digest tag individually and then copying all tags with no clobber afterwards to pick up any new tags. The individual copy is very slow compared to the all tags copy due to it running as a single operation in the shell.

One idea would be to support the --omit-digest-tags in combination with the --all-tags for crane copy but this doesn't quite seem right, maybe a --all-non-digest-tags flag would work?

Alternatively a copy option that checks if the digests match before pushing might also work.