KIP-1052 — Enable warmup in producer performance test

Accepted Kafka 4.2 ProducerTesting

Adds --warmup-secs and --warmup-records options to kafka-producer-perf-test.sh to exclude the high-latency initialization phase from steady-state measurements. Without a warmup phase, performance test results included startup latency spikes from producer initialization and ISR establishment, requiring multi-hour test runs to dilute the startup noise below 1% of the measurement window.

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