NIFI-16046: Initial implementation for Processor / Connector Backlog

[markap14]

Summary

NIFI-00000

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[pvillard31]

I'll try to spend some time actually testing it but it LGTM from a code implementation PoV, a couple of comments that could be follow-up:

• The cluster merger (BacklogRequestEndpointMerger.mergeEntities) re-implements the same precision/sum/earliest-timestamp rules as Backlog.plus(). A shared helper that both the API and the DTO layer delegate to would avoid any potential drift.
• The API says calls are ~1/min and implementations may cache. For an expensive source query (a Kafka Admin round-trip, a Postgres WAL diff), it would help to have a small framework-provided TTL cache or at least a documented pattern, so every new implementer doesn't reinvent it and accidentally hammer the source.