KIP-615 — add ConstrainedCooperativeStickyAssignor

Discarded Kafka 2.4.2 Consumer

Proposes a `ConstrainedCooperativeStickyAssignor` that trades perfect balance for faster assignment computation when the number of consumers and partitions is very large. The existing `CooperativeStickyAssignor`'s algorithm becomes prohibitively slow (2.5+ minutes reported) with 2100+ consumers due to its O(n²) complexity in optimizing for both stickiness and perfect balance simultaneously.

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