KIP-1251 — Assignment epochs for consumer groups

Accepted Kafka 4.3 ConsumerProtocol

Adds an assignment epoch to the new consumer group protocol (KIP-848) so that OffsetCommit and AddOffsetsToTransaction requests can be validated against the member's current assignment, fencing zombie commits from members whose partitions have been reassigned. The KIP-848 protocol uses member epochs for heartbeat fencing but did not carry epoch information in offset commit or transactional offset commit paths, leaving a window for stale members to overwrite offsets of their successors.

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