[Feat]: Support ingesting indexes tagged with the referrers tag schema

[andaaron]

Is your feature request related to a problem? Please describe.

In case a server does not use the referrers API, the spec describes an equivalent workflow managed from client side:
https://github.com/opencontainers/distribution-spec/blob/main/spec.md#unavailable-referrers-api

Basically the client will manage a tag containing the subject hash in the name, which points to an index which contains the data which would have been returned by the referrers API.

zot currently supports the referrers API, but:

• there are clients who may elect not to use it (disabled by default in latest notation version and release v1.0.0)
• there are artifacts sync-ed from servers which do not support it

In these cases zot receives these already existing indexes tagged using the https://github.com/opencontainers/distribution-spec/blob/main/spec.md#referrers-tag-schema

There are some issues with current implementation:

• the logic writing this data in MetaDB for notation signature is bugged, resulting in MetaDB erroring when reading this index, which results in GlobalSearch returning errors
• there are issues with sync-ing this index, probably the same as for notation, but they need to be investigated

Describe the solution you'd like

The notation signature should correctly be updated in MetaDB when pushing a Notation signature with the referers schema disabled client-side.
The notation signatures should correctly be synced and updated in MetaDB from a server already having an index.
We should sync the index tagged with the referrers schema as well.

Describe alternatives you've considered

No response

Additional context

There is currently a PR which is intended to fix these issues (at a minimum for notation side): #1882

[andaaron]

Done in #1882