KIP-979 — Allow independently stop KRaft processes

Accepted Kafka 3.7 KRaftAdmin

Adds separate PID files and stop scripts for KRaft controller and broker processes so they can be independently stopped when co-located on the same machine. Previously a single `kafka-server-stop.sh` script signaled all Kafka processes indiscriminately, making it impossible to stop just the controller without killing the co-located broker.

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