feat(bigframes): Support groupby.agg/transform with udf transpiler

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request introduces subscripting support (via GetItemOp and DynamicGetItemOp) across Ibis, Polars, and SQLGlot compilers for arrays, structs, and strings. It also enables transpilation of custom Python functions (UDFs) within GroupBy aggregations and transformations. The review feedback highlights several critical runtime issues, including missing imports (bigframes and window_specs) that will cause NameErrors, and unhandled string-like inputs in the Polars implementation of GetItemOp.