KAFKA-5761: Serializer API should support ByteBuffer

[flore77]

This implements KIP-646

Summary

• Adds serializeToByteBuffer() default method to the Serializer interface, returning ByteBuffer instead of byte[]
• Overrides in built-in serializers (ByteArray, ByteBuffer, Bytes, Integer, Long, Short, Float, Double, Boolean, Void) with optimized implementations that avoid unnecessary copies
• Updates the producer pipeline (KafkaProducer → RecordAccumulator → ProducerBatch) to use ByteBuffer end-to-end, eliminating byte[] → ByteBuffer wrapping in MemoryRecordsBuilder
• Adds ByteBuffer overload to Partitioner interface with zero-copy optimization for exact backing arrays
• Extends Utils.murmur2() and BuiltInPartitioner.partitionForKey() to accept ByteBuffer natively, with byte[] versions delegating to them

Test plan

• SerializationTest — verifies serializeToByteBuffer() matches serialize() output for all built-in serializers, null handling, and ByteBuffer zero-copy
• PartitionerTest — verifies default ByteBuffer partition method passes backing array directly (zero-copy) and copies when buffer is a slice
• KafkaProducerTest — end-to-end producer pipeline with ByteBuffer path
• RecordAccumulatorTest, ProducerBatchTest, SenderTest, MockProducerTest — existing tests pass via byte[] overloads that delegate to ByteBuffer

[clolov]

Hello @flore77! As far as I have seen KIP-646 was never voted on and we can't really make a change before it passes a vote. If you are interested in taking the KIP forward I would first suggest you bump the DISCUSS thread so that people can have the opportunity to raise suggestions. Afterwards, open a VOTE thread and if that passes successfully we can come back to reviewing this PR 😊

[flore77]

@clolov ack. I was not aware of the voting process, I requested an ASF account to drive this KIP forward. Thanks for letting me know!

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