migrate "oci-image-tool validate" to umoci

[cyphar]

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).