Add SMALLEST and LARGEST functions in PyDough

[knassre-bodo]

Goal: add a functions to PyDough with the following functionalities, which should be tested e2e (see test_pipeline.py for examples):

• SMALLEST(X, Y, ...) is equivalent to the SQL function LEAST(X, Y, ...), or the Python min function when called on results from each record (as opposed to be being used as an aggregation).
• LARGEST(X, Y, ...) is equivalent to the SQL function GREATEST(X, Y, ...), or the Python max function when called on results from each record (as opposed to be being used as an aggregation).

Not every SQL dialect has LEAST or GREATEST, but the ones that don't generally allow MIN or MAX to take in 2+ arguments (in which case they do this behavior instead of aggregation). The way to handle this in SQLGlot is to convert to a 2+ argument call to MIN or MAX. For ANSI, a fallback convert to a convoluted CASE statement can be done (e.g. LEAST(a, b, c) => CASE WHEN a <= LEAST(b, c) THEN a ELSE LEAST(b, c) END with LEAST(b, c) recursively transformed in a similar manner). Both functions should throw an error if called on <2 arguments.

These features need to be documented in the function list documentation