Support OCI Artifacts and reference types

[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 issue does not track the updates/changes to ORAS artifacts or API due to the above and should only focus on making sure that the ORAS project ensures that it implements the OCI specification.

[sajayantony]

Both PRs are not merged and OCI has an release candidate.
Oras should be able to consume the specification and fully unblocked -

https://github.com/opencontainers/image-spec/tree/v1.1.0-rc1
https://github.com/opencontainers/distribution-spec/tree/v1.1.0-rc1

[shizhMSFT]

Closed by #271