Enumerate Python source code into text files

We currently don't store the Python source code anywhere that UDFs are defined with. Store this source code into text files and enumerate them (eg `function_1`, function_2`, etc). So that loading & registering UDFs is easier.

Start by tracing where Python UDF definitions are handled and where UDFs are loaded and registered; the issue does not name specific files or tests. Done means the Python source is stored in text files with enumerated names such as function_1 and loading and registering UDFs can use those files.

Written by the indexing model from the issue text.