Referrers API support for OCI Layout

[shizhMSFT]

The OCI layout can be viewed as a simplified local repository, which should also support the equivalence of Referrers API.

Proposal: Let content/oci.Store and content/oci.ReadOnlyStore implement registry.ReferrerFinder.

[shizhMSFT]

In terms of functionality, Referrers() is a subset of Predecessors(). Therefore, we can use Predecessors() to implement Referrers().

[shizhMSFT]

There is a generic implementation in oras: https://github.com/oras-project/oras/blob/660d167c8434b19da455828e1f30ee7f3864638c/internal/graph/graph.go#L65

We can consider move and improve the above implementation to oras-go.