feat(bigframes): Enable local udf execution

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request adds support for struct operations, JSON string conversion, and Python UDF compilation in the Polars compiler, alongside refactoring UDF signature resolution into a shared utility and adding corresponding unit tests. The review feedback highlights a missing from __future__ import annotations import in _utils.py needed for PEP 604 type hints on older Python versions, an incorrect struct field extraction syntax in Polars, and a performance bottleneck in the UDF compiler where callable compilation is performed per-row instead of once.