KIP-512 — make Record Headers available in onAcknowledgement and onComplete

Accepted Kafka 4.1 ProducerClient

KIP-512 exposes record headers in the `ProducerInterceptor.onAcknowledgement()` callback alongside the existing topic, partition, offset, and timestamp. Without headers, use cases like latency measurement (correlating request/response via header IDs) and distributed tracing (propagating trace context) cannot be implemented in producer interceptors.

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