[flink] Support custom lookup shuffle for Flink lookup join.

[naivedogger]

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• I searched in the issues and found nothing similar.

Motivation

Fluss's Flink lookup join doesn't custom-shuffle the probe stream, so probe rows are distributed to lookup subtasks arbitrarily. This hurts lookup.cache (partial) hit rate and enlarges RPC fan-out, since each subtask may hit any bucket / any TabletServer.
Flink 2.0 added SupportsLookupCustomShuffle (FLIP-462), which lets a source define the probe-stream distribution. We can shuffle by bucket key so each subtask handles a stable set of buckets → better cache locality and lower RPC fan-out.

Solution

• Implement SupportsLookupCustomShuffle and provide an InputDataPartitioner that maps the lookup key to a Fluss bucket using the existing bucketing logic (consistent with the write path / PrimaryKeyLookuper).
• Only enable when the lookup key covers the bucket key (PK lookup / prefix lookup on bucket key); otherwise fall back to default distribution.
• Note: the interface is Flink 2.0+, so this targets fluss-flink-2.2 only (FlinkTableSource lives in fluss-flink-common which must stay Flink 1.18 compatible).

Anything else?

No response

Willingness to contribute

• I'm willing to submit a PR!

[naivedogger]

I'd like to take this.