Support OCI Artifacts and referrers API

[sajayantony]

As the reference type working group has submitted the PR to OCI, opencontainers/image-spec#934 and opencontainers/distribution-spec#335 ORAS needs to support the implementation of these media types and APIs

This is dependent on - #267

This issue does not track the issue for change/updates of ORAS artifacts or API and should only focus on making sure that the ORAS project ensures that it implements the OCI specification.

[shizhMSFT]

Closed by #271