KIP-1203 — Allow to configure custom `ReplicaPlacer` implementation

Discussion Broker

Allows operators to plug in a custom ReplicaPlacer implementation via a new unstable broker config, bypassing the default StripedReplicaPlacer round-robin algorithm. The default round-robin placement cannot be overridden without code changes, but some advanced use cases such as internal testing or specialized AZ-aware scenarios require custom placement strategies.

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