KIP-944 — Support async runtimes in consumer

Discarded Consumer

Proposes allowing consumer callbacks (e.g., `onPartitionsRevoked`) to call consumer APIs from a different thread than the poll thread, enabling async runtimes to commit offsets from within rebalance callbacks without deadlocking. The current threading model forbids re-entrant consumer calls, making it impossible to do work in rebalance callbacks with async frameworks.

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