Add SIGN function in PyDough

[knassre-bodo]

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

• SIGN(X) is equivalent to the python string expression 0 if X == 0 else (1 if X > 0 else -1), or the SQL expression CASE WHEN X == 0 THEN 0 WHEN X > 0 THEN 1 WHEN X < 0 THEN -1 END

This feature need to be documented in the function list documentation