support local/package level and serializable python modules/macros

[woods-chen]

resolves #10078

Suppose that we need to remove (dedent) the same number of leading whitespaces in each line of a sql command or a string of documentation. I've been googling for jinja solutions, however the solutions are quite complicated. And it would be quite ease if we can invoke a python module/function.
The proposed method is, we can write a macro in a python file "macros/my_python_module.py":

from textwrap import dedent

and then we call call the defined module/function in our model "models/test1.sql":

{% set query_cmd %}
    select col_1,
              col_2
    from table_name
    where col3=1
        and col4=2
{% endset %}
{{ my_python_module.dedent(query_cmd) }}

What's more, as the implementation of this python macro is within the project locally, we can call the macro using my_project.my_python_module.dedent if this project is imported to other projects as a package, just as what we do with a jinja macro.

Problem

Solution

add a PythonModule node class in additional to the Macro node class, which can be parsed and serialized.
However there are some limitations:

1. the python macro is in project level which is the same as jinja macros, and should be called with the heading package name if we need to call the python macro in other projects.
2. Each directory (python module) should have a "__init__.py" file inside except for the "macros" directory.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[dbeatty10]

Closing per #10078 (comment)