KIP-514 — Add a bounded flush() API to Kafka Producer

Discussion Producer

Adds a `Producer.flush(long timeout, TimeUnit unit)` overload that blocks until all buffered records are sent or the timeout expires, throwing `TimeoutException` on expiry. The existing `flush()` has no timeout, meaning it can block indefinitely if the broker is slow or network connectivity is degraded, giving callers no ability to bound flush duration.

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