Add missing scalar functions (union, arrow metadata, get_field, version, row)

[timsaucer]

Summary

Several miscellaneous scalar functions from upstream DataFusion are not exposed in datafusion-python.

Missing Functions

Union type functions:

• union_extract — extracts a value from a union type by field name
• union_tag — returns the tag (active field name) of a union type

Arrow metadata/cast functions:

• arrow_metadata — returns Arrow field metadata
• arrow_try_cast — attempts to cast an expression, returning null on failure instead of erroring

Other:

• get_field — extracts a field from a struct by name
• version — returns the DataFusion version string
• row — alias for the struct constructor

Upstream Reference

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