Add `ostree admin post-copy` command

Add `ostree admin post-copy` command

This command will apply fs-verity on all objects that need it and
needs to be called when an ostree deployment has been copied on a
file-by-file basis, which would loose information such as fs-verity.

This is needed by osbuild which works by creating the final image in a
rootfs, and then separately copying that rootfs file-by-file to a
loopback mounted filesystem image.