KIP-872 — Add Serializer#serializeToByteBuffer() to reduce memory copying

Accepted ClientProducer

Adds Serializer#serializeToByteBuffer() as a default method returning ByteBuffer, enabling zero-copy serialization for types backed by ByteBuffer and eliminating the intermediate byte[] allocation and wrapping in KafkaProducer#doSend(). The current path always allocates a byte[], wraps it in a ByteBuffer, then writes it into MemoryRecordsBuilder, incurring unnecessary heap pressure for ByteBuffer-based payloads.

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