Add support for OCI Image Manifest

[chantra]

Image like s390x/ubuntu uses OCI Image Manifest, which per https://github.com/opencontainers/image-spec/blob/v1.0.1/manifest.md is very similar to vnd.docker.distribution.manifest.v2.

Add a test to confirm we can deserialize an OCI Image Manifest as a ManifestSchema2Spec.

Also add a test to confirm that we can now fetch the manifest from docker.io. Previously it would fail with MANIFEST_UNKNOWN.

This is built on top of #260 . I stacked it because it would otherwise have merge conflicts due to the tests.

[chantra]

As a follow up, I am happy to revive #244 if @luizribeiro can't get to it.

[chantra]

@PratikMahajan if you could look into this one too, that would be great.