Expose `referrers` on `ContainerRepositoryTagType`

Why are we doing this work

To support presentation of image signatures on the frontend we need to expose the referrers for a tag. This will allow the frontend to see what signatures are associated with a tag.

Relevant links

Non-functional requirements

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

Implementation plan

1. Add a field referrers onto Types::ContainerRepositoryTagsType
2. When this field is requested, pass the referrers=true param to the internal tags API (exact implementation TBD on #411387 (closed))
3. return the referrers data from the internal tags API

This field will not be available when using the V2 tags API, and should return an empty array in that case.

Verification steps