Notation CLI supporting OCI reference type

[yizha1]

Notation CLI depends on Notation-go to support OCI reference type. The Notation-go work item is tracked here at notaryproject/notation-go#136. Besides the update in Notation-go, this issue tracks any other tasks of notation CLI to support OCI reference type.

[iamsamirzon]

@yizha1 - I think this item and notaryproject/notation-go#136 should both be for RC.1.

cc: @vaninrao10

[FeynmanZhou]

Per today's community call, Notation will migrate to the OCI artifact manifest from the ORAS artifact manifest in RC.1 and the fallback solution is planned in RC.2.

I am concerning there are still less registries supporting the OCI artifact manifest when RC.1 is available at the end of this month, which means the Notation usability will be broken.