KIP-166 — Add a tool to make amounts of replicas and leaders on brokers balanced

Discussion AdminBroker

Proposes a `kafka-replica-balancer.sh` tool that rebalances the distribution of partition replicas and preferred leaders evenly across all brokers. After broker additions, decommissions, or rolling restarts, replicas become imbalanced (some brokers hold more leaders than others), degrading throughput; the existing `kafka-preferred-replica-election.sh` only elects preferred leaders without rebalancing replica assignment.

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