[Feature]: Support record referrers deletion

[paralta]

Problem Statement

The Store service has PushReferrer and PullReferrer but no way to delete referrers. Users cannot:

• Remove a single referrer (e.g. one signature).
• Remove all referrers of a given type for a record.
• Remove all referrers for a record

Proposed Solution

• Add DeleteReferrer RPC: stream DeleteReferrerRequest, with optional referrer_type (scope by type) and referrer_digest (delete a single referrer by manifest digest).
• Implement in the store layer.

Note

After PR #985 is merged, deleting a referrer (especially a signature) should mark the record for signature verification so the reconciler can recompute trusted status.

Alternatives Considered

No response

Additional Context

No response

Checklist

• I have read the contributing guidelines
• I have verified this does not duplicate an existing feature request

[csirmazbendeguz]

I've looked into this and it's not clear to me WHY would someone want to delete referrers?

Doesn't this contradict the Public Auditability and Non-Repudiation principles of the system?

https://spec.dir.agntcy.org/draft-mp-agntcy-ads.html#name-content-provenance-and-digi

Let's talk, maybe I'm missing something