Provide more signature details in `status`

[jlebon]

I just started signing my bootc image for my workstation and using --enforce-container-sigpolicy, and expected to see in the status output something similar to rpm-ostree's "Valid signature by ...".

I suspect that would require work in the skopeo proxy to be able to also get the signature info out of c/image when pulling an image and storing that somewhere under /var. Or store it in whatever usual place c/storage stores it (e.g. /var/lib/containers/sigstore), though that might be awkward to do since we're not actually importing into c/storage. (Obviously related to #20.)

[cgwalters]

I suspect that would require work in the skopeo proxy to be able to also get the signature info out of c/image

Yes...I think the status quo is semi-intentional on the container-libs side that signatures are only validated at pull time. There's an issue/PR about this there somewhere.