KIP-957 — Support Async runtimes

Discarded Consumer

Proposes relaxing the `KafkaConsumer`'s single-thread reentrance check (`acquire()`) to allow callbacks (e.g., `onPartitionsRevoked`) to call back into the consumer from a different thread, enabling async runtimes (Kotlin coroutines, ZIO) to perform offset commits inside rebalance callbacks. The current strict thread-ownership check in `acquire()` makes it structurally impossible for async runtimes to call `consumer.commitSync()` from within a rebalance listener without deadlocking or triggering a `ConcurrentModificationException`.

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