Adds ability for action to specify its own source code

[elijahbenizzy]

This enables people to build classes that have different sources (E.G. function wrappers)

Changes

• added the def get_source_code() function

How I tested this

• unit test + manually

Notes

• We'll want to do this with Hamilton + langchain

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.