[FLINK-40191][python] Add aggregation support to PyFlink DataFrame API

[auroflow]

What is the purpose of the change

This pull request adds global and grouped aggregation support to the PyFlink DataFrame API, as described in FLINK-40191.

It allows users to aggregate an entire DataFrame with DataFrame.agg(), or group rows with DataFrame.group_by() and aggregate them with GroupedDataFrame.agg().

Brief change log

• Add DataFrame.agg() for global aggregations.
• Add DataFrame.group_by() and GroupedDataFrame.agg() for grouped aggregations.
• Export GroupedDataFrame from pyflink.dataframe.
• Add API documentation and usage examples.

Verifying this change

This change added tests and can be verified as follows:

• Added unit tests for global and grouped aggregation schemas and output ordering.
• Added unit tests for Python argument validation and planner-delegated validation.
• Added a batch integration test covering grouped sum and count aggregations.

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? PyDocs and Python API docstrings

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CI report:

• 98bc2a6 Azure: SUCCESS

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