KIP-34 — Add Partitioner Change Listener to Partitioner Interface for Multiple Use Case

Discussion Producer

KIP-34 proposes adding a partition-change listener callback to the `Partitioner` interface so custom partitioners are notified when a topic's partition count changes or partitions go offline/online. Without this, a custom partitioner relying on partition count (e.g. to enforce a buffer cap) has no way to react to topology changes between metadata refresh intervals.

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