Add missing conditional functions (greatest, least, nvl2, ifnull)

[timsaucer]

Summary

Several commonly-used conditional functions from upstream DataFusion are not exposed in datafusion-python.

Missing Functions

• greatest — returns the greatest value from a list of expressions
• least — returns the least value from a list of expressions
• nvl2 — returns one value if input is not null, another if it is null
• ifnull — alias for nvl/coalesce

Upstream Reference

• https://datafusion.apache.org/user-guide/sql/scalar_functions.html (Conditional Functions section)

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.