KIP-369 — Alternative Partitioner to Support "Always Round-Robin" Selection

Accepted Kafka 2.4 Producer

Introduces `RoundRobinPartitioner` as an alternative partitioner that distributes records with null keys evenly across all partitions in strict round-robin order, regardless of sticky batching behavior. The default partitioner applies sticky batching which clusters null-key records into a single partition per batch for throughput, but applications where ordering guarantees are absent and even distribution is required (e.g., multi-DC fan-out) cannot opt out.

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