KIP-505 — Add new public method to only update assignment metadata in consumer

Discussion Consumer

KIP-505 adds a `Consumer.updateAssignmentMetadata(Duration timeout)` public method that triggers partition assignment, offset fetching, and group coordination without returning any records. The `poll(0)` hack used for this purpose has undefined semantics in the new `poll(Duration)` API (a zero duration may not provide enough time for the coordinator round-trip), and there is no dedicated no-fetch method for callers that only need assignment state to be current.

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