[python] Support VARIANT type in pypaimon

[chenghuichen]

Purpose

Paimon Java already supports the VARIANT type, but pypaimon has no equivalent implementation. This PR adds VARIANT read/write support to pypaimon, enabling compute engines to integrate it. For example, Daft is planning native VARIANT support at the engine level.

Tests

• Unit tests
  • pypaimon/tests/variant_test.py
• E2E tests
  • run_java_variant_write_py_read_test
  • run_py_variant_write_java_read_test

[chenghuichen]

The right long-term solution is to wait for PyArrow's official Variant support (apache/arrow#45937), which would guarantee both correctness and performance from the upstream ecosystem. We should continue tracking that progress.

That said, a Python-based implementation now is a reasonable short-term step: users and compute engines can begin integrating VARIANT support early, and as long as GenericVariant's interface is designed robustly, the underlying implementation can be swapped for PyArrow's later without breaking users.