Support tag + digest image reference

[cgwalters]

Currently

$ bootc switch quay.io/centos-bootc/centos-bootc:stream10@sha256:9735171dc889739be0ee60621b1ee2f74811ab1eaa54b4f63b5a662c1ebe2949
ERROR Switching: Pulling: Creating importer: failed to invoke method OpenImage: failed to invoke method OpenImage: Docker references with both a tag and digest are currently not supported
$

This is coming from skopeo

$ skopeo inspect docker://quay.io/centos-bootc/centos-bootc:stream10@sha256:9735171dc889739be0ee60621b1ee2f74811ab1eaa54b4f63b5a662c1ebe2949
FATA[0000] Error parsing image name "docker://quay.io/centos-bootc/centos-bootc:stream10@sha256:9735171dc889739be0ee60621b1ee2f74811ab1eaa54b4f63b5a662c1ebe2949": Docker references with both a tag and digest are currently not supported 

Yet

$ podman pull  quay.io/centos-bootc/centos-bootc:stream10@sha256:9735171dc889739be0ee60621b1ee2f74811ab1eaa54b4f63b5a662c1ebe2949
...

works just fine.

There's a lot of prior discussion of this, see containers/image#1736

AFAICS from the change to cri-o here cri-o/cri-o@c862e1f the idea is all c/image consumers need to do the tag-stripping first, which is presumably what podman is doing too.

So I guess we should also just do that. To implement this, we basically need to do that tag stripping before we pass the target image to containers-image-proxy.

To be clear the implementation here is that the tag is purely for human consumption, it's not validated (beyond syntactically).

Note do want to retain the original full tag+digest form so it appears in bootc status.

[cgwalters]

xref coreos/zincati#1272 (comment) for one motivation but this would be really obviously useful in general.