KIP-1299 — Use key range in ProducerPerformance

Accepted Kafka 4.4 ProducerTesting

Adds key distribution support to kafka-producer-perf-test via --key-distribution (none|random|fixed) and --key-range parameters, enabling benchmarks that produce keyed messages. The tool previously always used null keys, making performance results systematically optimistic and unable to reflect real workloads that depend on semantic partitioning, stream joins, or cross-cluster replication.

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