(feat) function calling incl. unit tests (first draft)

[tobitege]

Implement function calling with file_ops methods as an example.

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions

I'm 100% sure that OH should offer such a feature, but I'm architecturally not quite sure how to combine it best with the current base.
How the files ended up is somewhat how the LLM's suggested it, and I'm pretty sure, there'll be changes needed.
There's an added test_llm_tools.py file with 10 tests.

cc @enyst @xingyaoww what do you guys think?

This replaces #4525 as this has the branch in repo to be editable here.

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https://litellm.vercel.app/docs/completion/function_call

[tobitege]

For initial discussion comments see #4525