add a version number API

[cgwalters]

Today we hardcode looking up and injecting a label key version; I did this because it's what's used by e.g. RHEL UBI (and the fedora/centos images). However...the OCI spec calls instead for org.opencontainers.image.version so arguably we should use that.

(That key is used by e.g. registry.opensuse.org/opensuse/leap:latest)

But then I was going to look at what docker.io/library/busybox does and it's neither? I guess they just rely on tags-as-versions.

(There is also apparently the label schema stuff that predated OCI but I am not sure anyone uses that today)

(Except there's a "DockerVersion": "20.10.12" thing...what the heck is that? It seems to be on many images on the Docker Hub?)

Edit: DockerVersion is something that seems to be injected by docker build or so, it's not related to this.

Edit: xref containers/oci-spec-rs#118