opencontainers
migrate "oci-image-tool validate" to umoci
This is the last step necessary to complete umoci's inclusion in the OCI -- to move validation into a more widely-known project so it can be used by more people. This does have the slight downside that the validation and reference implementation of the image-spec will live in the same repository but I don't think there's a much nicer solution (barring maintenance of image-tools to be revitalised).
This is now something we have to do because image-spec changes have made the old oci-image-tool runs stop working, and it's probably easier to just sit down and implement this. I took a quick look and it's not actually that bad (compared to oci-runtime-tool, we basically just have to use something like github.com/santhosh-tekuri/jsonschema/v6...).