Add missing aggregate functions (grouping, percentile_cont, var_population)

[timsaucer]

Summary

A few aggregate functions from upstream DataFusion are not yet exposed in datafusion-python.

Missing Functions

• grouping — returns an integer indicator for which grouping set a row belongs to (used with GROUPING SETS, CUBE, ROLLUP)
• percentile_cont / quantile_cont — computes a percentile value using continuous interpolation
• var_population — alias for var_pop

Upstream Reference

• https://datafusion.apache.org/user-guide/sql/aggregate_functions.html

Implementation

• Rust bindings: crates/core/src/functions.rs
• Python wrappers: python/datafusion/functions.py

Note: This gap analysis was performed using an AI agent comparing upstream DataFusion v53 documentation against the current datafusion-python codebase.