Implement OCI 1.1 referrals index

Why are we doing this work

We would like to implement the OCI 1.1 API for listing referrers. This should only be done after we are able to handle writes.

Relevant links

OCI Distribution Spec on enabling referrers API

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

TBD

Verification steps