[FLINK-38491][pipeline-connector][iceberg] fixed iceberg compaction parallelism

[fcfangcc]

1. sink.compaction.commit.parallelism not expose to user.
2. compaction partitionCustom Logic error,( bucket % numPartitions ) maybe negative

java.lang.ArrayIndexOutOfBoundsException: Array index out of range: -1
at org.apache.flink.runtime.io.network.partition.BufferWritingResultPartition.appendUnicastDataForNewRecord(BufferWritingResultPartition.java:290)
at org.apache.flink.runtime.io.network.partition.BufferWritingResultPartition.emitRecord(BufferWritingResultPartition.java:156)
at org.apache.flink.runtime.io.network.api.writer.RecordWriter.emit(RecordWriter.java:108)
at org.apache.flink.runtime.io.network.api.writer.ChannelSelectorRecordWriter.emit(ChannelSelectorRecordWriter.java:55)
at org.apache.flink.streaming.runtime.io.RecordWriterOutput.pushToRecordWriter(RecordWriterOutput.java:140)
at org.apache.flink.streaming.runtime.io.RecordWriterOutput.collectAndCheckIfChained(RecordWriterOutput.java:120)
at org.apache.flink.streaming.runtime.io.RecordWriterOutput.collect(RecordWriterOutput.java:101)
at org.apache.flink.streaming.runtime.io.RecordWriterOutput.collect(RecordWriterOutput.java:53)
at org.apache.flink.streaming.api.operators.CountingOutput.collect(CountingOutput.java:60)
at org.apache.flink.streaming.api.operators.CountingOutput.collect(CountingOutput.java:32)
at org.apache.flink.streaming.runtime.operators.sink.CommitterOperator.emit(CommitterOperator.java:199)

In java -5 % 4 = -5 - (4 * (-1)) = -5 + 4 = -1

[yuxiqian]

Thanks for @fcfangcc's nice work! Just left some comments.