Provide more signature details in `status`

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 https://github.com/bootc-dev/bootc/issues/20.)