Add missing date/time functions (current_timestamp, date_format, make_time)

[timsaucer]

Summary

A few date/time functions from upstream DataFusion are not exposed in datafusion-python.

Missing Functions

• current_timestamp — returns the current timestamp
• date_format — formats a date/timestamp as a string
• make_time — creates a time value from hour, minute, and second components

Upstream Reference

• https://datafusion.apache.org/user-guide/sql/scalar_functions.html (Time and Date 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.