Migrate to OCI Artifact from ORAS Artifact

[shizhMSFT]

We will migrate to OCI Artifact from ORAS Artifact once the following PRs are merged.

• Working Group Proposal for Reference Types opencontainers/image-spec#934
• Working Group Proposal for Reference Types opencontainers/distribution-spec#335

Tasks:

• Migrate artifactspec.Descriptor to ocispec.Descriptor #307
• Add support for OCI Artifact manifest #308
• Merge oras.PackArtifact() into oras.Pack() #309
• Improve performance of filters of oras.ExtendedCopy() caused by information loss due to descriptor conversions #310
• Update implementation of Referers API #311
• Add client-side compatibility for pushing manifest with subject field #312
• Add client-side compatibility for deleting manifest with subject field #347

This issue covers #267 and #268.

[sajayantony]

Should we start a new branch for OCI now that you have go structs for OCI. Similarly, for oras - CLI?
/cc @nima @michaelb990 @shizhMSFT

[rdpsin]

Any update on this work?

[Wwwsylvia]

@rdpsin We will be working on this after releasing oras-go v2.0.0-rc3, which is scheduled for Sep. 5th.

[SteveLasker]

Just a clarification that we'll need to maintain support for ORAS artifacts, until all registries have migrated to the OCI Artifact spec.

1. Check if /referrers is supported (best future case)
2. Check if _oras/referrers is supported (limited time, as ACR, ECR and Zot migrate to /referrers)
3. Fallback to using OCI Image manifest, with a refers/subject property (see Rename refers field to subject #950)