[FLINK-38740][table] Introduce Welford's online algorithm for variance related functions to avoid catastrophic cancellation in naive algorithm

[dylanhz]

What is the purpose of the change

Fix catastrophic cancellation in naive algorithm of variance related functions.

Brief change log

• Introduce Welford's online algorithm for variance related functions.
• Add a new internal built-in agg function WELFORD_M2.
• Add new rewrite rule to reuse the intermediate result welford_m2 for all variance related functions.

Verifying this change

New test class: MathAggFunctionITCase.
Update some existing tests using variance related functions.

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? (no)
• If yes, how is the feature documented? (not documented)

[dylanhz]

The first commit will be removed once it is merged.
#27319

[flinkbot]

CI report:

• c5c78d5 Azure: SUCCESS

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