Add conformance test coverage for data field

[imjasonh]

The data proposal was merged: https://github.com/opencontainers/image-spec/pull/826 🎉

This adds some conformance test coverage that the new field is round-trippable through registries, helpfully piggybacking off of #311. Since registries don't validate that the contents match it doesn't seem fair to add that case to conformance, but it's something to consider in the future.

@jdolitsky

[jdolitsky]

fire. let me test this out and report back

[SteveLasker]

Adding #321 for tracking

[jdolitsky]

@opencontainers/distribution-spec-maintainers - can we get another +1?

[imjasonh]

Friendly ping.