[FLINK-40419][python] Add regular join support to DataFrame API

[SulphurFH]

What is the purpose of the change

Add regular relational join support to the PyFlink DataFrame API as proposed in FLINK-40419.

Brief change log

• Support inner, left, right, full/outer, semi, anti, and cross joins.
• Support shared, differently named, computed, and expression-based join keys.
• Validate invalid argument combinations and duplicate output columns.
• Add planner and batch execution tests.

Verifying this change

This change added tests and was verified against a locally built Flink 2.4-SNAPSHOT distribution:

• DataFrameJoinTests: 9 passed.
• Join batch integration tests: 4 passed.
• Full pyflink.dataframe test suite: 351 passed, 1 skipped.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): yes
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? Python API docstring

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• Yes (please specify the tool below)

Generated-by: Codex GPT-6

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CI report:

• 5a29e52 Azure: SUCCESS

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