KIP-89 — Allow sink connectors to decouple flush and offset commit

Accepted Kafka 1.0 Connect

KIP-89 adds a `SinkTask.preCommit()` method and a `SinkTaskContext.requestCommit()` method to decouple offset commits from periodic buffer flushes in Kafka Connect sink connectors. Previously, the runtime called `flush()` on the fixed `offset.flush.interval.ms` schedule and committed offsets immediately after, forcing connectors that manage their own buffering (size-based, time-based, or not at all) to flush unnecessarily.

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