feat: update rust to pip install code block requirements

[starpit]

• adds a requirements attribute to CodeBlock
• does not update the pdl_interpreter logic to deal with it
• instead, updates the rust logic to scan the program and pip install those requirements
• for now, those requirements are installed into the main venv used by the interpreter

TODO

• refine the rust logic to rewrite the code blocks so as to utilize the venv created for that requirements
• then undo the hacks put in place in this PR so that we have a separate venv for each distinct requirements
• see feat: keep rust-managed code block venvs separate #755

The thought process here:

Logic: code-generate a new program such that each code block with a requirements attribute is rewritten to:

1. remove the requirements attribute as it is no longer needed
2. write the python code to a temp file, call it /tmp/logic.py for this discussion
3. update the code attribute to look like

command: /path/to/venv/bin/python /tmp/logic.py

This should allow us to use the pdl_interpreter, where it is none the wiser of our venv caching.

[jgchn]

This should be only enabled for lang: python. Should we check for it?

[starpit]

nice catch. do you know how to express this in pydantic?

[starpit]

this issue seems to indicate that we could do it, but only with a custom model validator pydantic/pydantic#10252

[starpit]

ok i added a model validator. thanks for the good catch!

https://github.com/starpit/prompt-declaration-language/blob/pip-install-reqs/src/pdl/pdl_ast.py#L479-L485

[jgchn]

I was going to suggest Pydantic validator as well. Also wondering if requirements should actually be Optional[str | List[str]?

[starpit]

very good! done