Flink Sink V2: Add CommitGate plugin interface for deferred commits

[herbherbherb]

Summary

Adds a CommitGate plugin interface to IcebergSink that controls whether committables are emitted downstream or buffered in Flink ListState. This enables use cases like pausing commits during catalog maintenance operations while keeping the Flink job running.

Resolves #15770.

Changes

• New: CommitGate.java -- @FunctionalInterface with a single method: boolean isCommitAllowed(long checkpointId)
• Modified: IcebergWriteAggregator -- accepts optional gate, adds ListState for buffering, gate check + buffer/flush logic in prepareSnapshotPreBarrier(), state initialization in initializeState()
• Modified: IcebergSink.Builder -- new commitGate() method, passed through to aggregator

Compatibility

• No behavioral change when the gate is not set (null default)
• Buffered committables are checkpointed in ListState, so recovery works correctly
• No changes to public API signatures of existing methods
• Fully backward compatible