feature: add support for OCI Artifacts (including GC)

[rchincha]

https://github.com/opencontainers/image-spec/blob/main/artifact.md

How would garbage collect work in this case?

[cyphar]

I suspect that the only thing that is necessary is to update image-spec to include the subject field, which is what allows artifacts to indicate the blob dependency graph that the GC should use. Because of how umoci implements blob walking (reflection), that should be enough. When this issue was opened IIRC there was no consensus around how references would work for artefacts, and the referrers API appears to be the solution that came out after all of that discussion.

Unfortunately, due to the fact that image-tools (and runtime-tools) is abandon-ware, we will first need to migrate the validation tools to umoci so that we can update the image-spec without failing CI.

(That being said, it seems that zot moved away from using umoci about a year ago... 😢)