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• A list of fully qualified class names implementing ConsumerGroupRebalanceListener.
• Classes are instantiated and configured by the broker at startup.
• Empty value means callback  feature is disabled feature is disabled

Note:  Dynamic updates are not planed in this scope out for now so that we can keep the KIP small, low-risk, and validate real usage first.

If adoption proves strong, we can add dynamic reconfiguration later without changing the callback interface.

Proposed Changes

Add a lightweight broker-side callback path for consumer-group rebalance events, without changing client APIs and without adding high-cardinality metrics.

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