[Flink] Support Flink batch insert into Fluss primary-key tables

[platinumhamburg]

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Description

Integrate the BulkLoad client SDK with the Flink connector so a bounded
INSERT INTO job can efficiently populate an empty Fluss primary-key table or
partition.

The Flink integration should coordinate one server transaction, build buckets
in parallel, and publish the target only after every bucket has completed.

Scope

• Add an explicit connector option for selecting the BulkLoad path.
• Enable BulkLoad only for bounded Flink jobs and supported primary-key tables.
• Begin one BulkLoad transaction for the physical target.
• Distribute the immutable build context to parallel build tasks.
• Assign every input row to its Fluss bucket.
• Build bucket snapshots in parallel through openBuildSession.
• Collect one completed result for every target bucket.
• Commit the complete manifest through one coordinated commit path.
• Abort or report a clear failure when the job cannot complete before the
  durable Commit decision.
• Keep temporary credentials out of Flink records, committables, checkpoints,
  and savepoints.
• Document configuration, supported cases, failure behavior, and operational
  limitations.

Willingness to contribute

• I'm willing to submit a PR!