Add STRIP function to PyDough

[knassre-bodo]

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

• STRIP(X) is equivalent to the python string expression X.strip(), or the SQL expression TRIM(X)
• STRIP(X, Y) is equivalent to the python string expression X.strip(Y), or the SQL expression TRIM(Y from X)

This feature need to be documented in the function list documentation