KIP-660 — Pluggable ReplicaPlacer

Discarded KRaftBroker

Introduces a pluggable `ReplicaPlacer` interface in the KRaft controller so operators can supply custom replica placement logic beyond the built-in rack-aware round-robin algorithm. The default round-robin placer is unaware of broker load, disk utilization, or network topology, leading to suboptimal initial assignments that then require expensive data movement via tools like Cruise Control.

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