Add support for fetching referrers and pushing referring manifests with a subject field

See
https://github.com/opencontainers/distribution-spec/blob/v1.1.0-rc1/spec.md#listing-referrers
https://github.com/opencontainers/distribution-spec/blob/v1.1.0-rc1/spec.md#pushing-manifests-with-subject

Solutions

• Add support in remote registry logic (Add referrers fetcher to remotes #7644)
• Add fetch support
• Add push support

Some other possibilities discussed are breaking out the remote registry logic into a server side plugin which transfer service could use but other plugins could also use as well (image validation, lazy-loading snapshotters, etc). This would allow referrers content to be fetched when needed rather than synchronized during fetch, but without each plugin having to worry about registry configuration.

Related #7592