KIP-568 — Explicit rebalance triggering on the Consumer

Accepted Kafka 2.6 Consumer

Adds a `KafkaConsumer.enforceRebalance(reason)` method that forces the broker to trigger an immediate rebalance by sending a `JoinGroupRequest` with `generation=-1`. Kafka Streams required the ability to force immediate rebalances for its cooperative protocol (task warm-up transfers, version probing), but the only existing mechanism—unsubscribe then resubscribe—was asynchronous and unreliable.

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