KIP-22 — Expose a Partitioner interface in the new producer

Accepted Producer

Exposes a Partitioner interface in the new Java producer, allowing custom partitioning logic to be injected via the partitioner.class configuration. The internal DefaultPartitioner (hash key, round-robin on null key) was not pluggable, preventing strategies like sticky partitioning or business-key-based routing without modifying application code to set partition manually.

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