KIP-694 — Support Reducing Partitions for Topics

Discussion AdminBroker

Proposes the ability to reduce the number of partitions for a topic via `AdminClient` and the broker, with data redistribution handled by merging trailing partitions into earlier ones during cleanup. Topics can only grow in partition count today, leading to ever-increasing partition totals in clusters with many short-lived or bursty topics that cause disk I/O degradation and increased failure blast radius.

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