Add OCI Artifact Support

[akashsinghal]

https://github.com/opencontainers/distribution-spec/blob/main/spec.md#listing-referrers

• Add OCI Artifact manifest type
• Add OCI Artifact manifest handlers
• Register v2/<repository>/referrers route
• Add filtering/paging support based on artifact type and page size

Open questions:

• do we move the ORAS artifact extended storage functionality into the main storage package?
• do we implement a separate storage interface for OCI Artifact referrers than the ORAS extension storage implementation?

[wangxiaoxuan273]

As I try to implement this feature, I've found the work to greatly exceed the scope of one pull request. Therefore I would like to split this task into multiple, manageable smaller pull requests as follows:

• [x] Add OCI Artifact manifest type | Update github.com/opencontainers/image-spec module to v1.1.0-rc2. This changes go.mod, go.sum and the vendor directory. #27
• [x] Add OCI Artifact manifest handlers, Part 1 | Implement manifest/ociartifact package to provide OCI artifact support. Unit tests are included. #29
• [x] Add OCI Artifact manifest handlers, Part 2 | Make changes to the registry/storage package to support OCI artifact manifests. Unit tests are included. #30
• [x] Add OCI Artifact manifest handlers, Part 3 | Make changes to registry/handlers/manifests.go so that the the handlers can handle OCI artifact manifests. I have no idea how to test this part. Probably this part will be tested when issue#21 is completely resolved. #31
• [x] Register v2/<repository>/referrers route, Part 1 | Implement the referrers route. Make changes to the registry/api/v2 package. #32
• [x] Register v2/<repository>/referrers route, Part 2 | Implement indexing referrers for artifact manifests. Make changes to the registry/storage package.#35
• [x] Register v2/<repository>/referrers route, Part 3 | Implement the referrersHandler for artifact manifests. #34
• [x] Register v2/<repository>/referrers route, Part 4 | Implement indexing referrers for image manifests. #41