Support the oras-project/artifacts-spec draft.1 reference types

Scope of release

• oras push --subject enabling the pushing of an oras.cncf.artifact.manfifest
• oras discover - listing the references
• Refactoring of oras and oras-go, pushing functionality into the libraries, with cli's in oras
  -- moved to Alpha 2
• Copying of a graph of artifacts, using the draft of the artifacts-spec