[python] Introduce rbm32 aggregator function

[PyRSA]

Purpose

This PR introduces the Python FieldRoaringBitmap32Agg implementations and aligns the Python roaring bitmap aggregation behavior with the Java implementation.

The changes include:

• Add rbm32 field aggregation support.
• Support roaring bitmap merge using serialized bytes representation.
• Add VARBINARY type validation for roaring bitmap aggregation.
• Add unit tests for aggregation and invalid input types.

Tests

• python -m pytest pypaimon/tests/test_field_aggregators.py::FieldRoaringBitmap32AggTest -q
• python -m pytest pypaimon/tests/test_aggregation_e2e.py -q
• python -m pytest pypaimon/tests/test_aggregation_merge_function.py -q
• git diff --check

[JingsongLi]

rbm64 is serialized using pyroaring.BitMap64, but it is incompatible with the Java Roaring64Bitmap format. Bidirectional verification fails in both cases: Java reads an EOF when accessing the Python datagram, and Python cannot deserialize the Java datagram; this breaks cross-engine reading between Java and PyPaimon.

[PyRSA]

rbm64 is serialized using pyroaring.BitMap64, but it is incompatible with the Java Roaring64Bitmap format. Bidirectional verification fails in both cases: Java reads an EOF when accessing the Python datagram, and Python cannot deserialize the Java datagram; this breaks cross-engine reading between Java and PyPaimon.

Thanks for the feedback. I confirmed that pyroaring.BitMap64 is not compatible with Java's Roaring64Bitmap serialization format, so cross-engine interoperability cannot be guaranteed. I'll remove the rbm64 aggregator from this PR and revisit it when a compatible implementation is available.

[JingsongLi]

Please rebase master.