KIP-1136 — Make ConsumerGroupMetadata an interface

Accepted Kafka 4.2 TransactionsConsumer

Makes ConsumerGroupMetadata an interface instead of a concrete class so that frameworks (e.g., Kafka Streams with EOS) can provide custom implementations without depending on the internal constructor. The class is passed to KafkaProducer.sendOffsetsToTransaction() to carry group metadata for transactional offset commits; making it an interface enables mock implementations in tests and custom metadata carriers in frameworks.

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