[GLUTEN-12298][FLINK] Support nexmark source multi-parallelism via ParallelSplit

[ggjh-159]

What changes are proposed in this pull request?

fix: #12298
depends: bigo-sg/velox4j#36、bigo-sg/velox#45

This PR consumes the ParallelSplit abstraction introduced by the companion velox4j PR:

• NexmarkSourceFactory.buildVeloxSource iterates over all per-subtask splits from getSplits(parallelism) and packs them into a single NexmarkConnectorSplit whose subtaskSplits carries one entry per subtask.
• GlutenSourceFunction.initSession detects ParallelSplit with parallelism > 1 and selects the per-subtask split via getSubtaskSplit(subtaskIndex, totalParallelism); otherwise it behaves as before.

How was this patch tested?

Manual run on a standalone Flink cluster with parallelism.default = 2, nexmark events.num = 10000, tps = 2000, query q0.

Signal	Before	After
Bid input rows	~18400 (duplicated)	9200 (no duplicates)
dateTime span	single timestamp	~5 seconds
Subtask splits	both share full range	subtask 0 firstEventId=1, maxEvents=5000; subtask 1 firstEventId=5001, maxEvents=5000

[lgbo-ustc]

Is there any test to cover this case?

[lgbo-ustc]

you need to update github workflow config to refer the right velox4j branch/commit

[lgbo-ustc]

for safty， subtask splits shoud use a non-ParallelSplit class

[ggjh-159]

Addressed. Split into two types:

• NexmarkConnectorSplit — leaf split, extends ConnectorSplit, only carries
  NexmarkGeneratorConfig config. Serialized across the Java/C++ boundary as before.
• NexmarkParallelSplit — container, extends ParallelSplit, holds
  List<NexmarkConnectorSplit> subtaskSplits. Only this type implements
  getSubtaskSplit, so a leaf split can no longer be mistaken for a parallel
  container.

NexmarkSourceFactory now builds a NexmarkParallelSplit wrapping the
per-subtask leaf splits, and the instanceof ParallelSplit check in
GlutenSourceFunction cleanly distinguishes container vs leaf.

[ggjh-159]

cc @lgbo-ustc UTs were added and the velox4j branch was set.

[lgbo-ustc]

@ggjh-159 update velox4j reference in ci

[ggjh-159]

@ggjh-159 update velox4j reference in ci

@lgbo-ustc done.

[lgbo-ustc]

@ggjh-159 CI failed. wait #12320 to be merged

[ggjh-159]

@ggjh-159 CI failed. wait #12320 to be merged

@lgbo-ustc The tests for this PR might also fail. I'll fix it as soon as possible.

[lgbo-ustc]

@ggjh-159 update the code again