[FLINK-34587][table] Introduce MODE aggregate function

[snuyanzin]

What is the purpose of the change

The PR introduces support of MODE aggregate function which returns the most frequent element in a group. NULL elements are ignored (as it is happening for other vendors), in case there is no non NULL elements in a group the result will be NULL. In case there are several elements with same frequency one of them will be returned (similar to other vendor's behavior)

Verifying this change

This change added tests and can be verified as follows: flink-table/flink-table-planner/src/test/scala/org/apache/flink/table/planner/runtime/batch/sql/OverAggregateITCase.scala flink-table/flink-table-planner/src/test/java/org/apache/flink/table/planner/functions/aggfunctions/ModeAggFunctionTest.java

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): ( no)
• 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? (not applicable )

[flinkbot]

CI report:

• 0d145325c9c7ca1d3f42e9007e8337b86fef5fde Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[affo]

@snuyanzin sorry, I completely missed this for a very long time. Do you still need my review?

[github-actions[bot]]

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