Typo in ReadMe + client_kwargs for OpenAI-compatible APIs in Detector + prompt on Decoder

[L3G5]

1. There was a small misprint in README.md. Should probably be import invariant.testing.functional instead of import invariant.testing.testing.functional as F. But I messed up the table of contents, so probably ReadMe fix should be discarded.
2. Added client_kwargs parameter for Detector() and InvariantString.extract(). It may be useful if people want to use local models through OpenAI-compatible API
3. Changed the system prompt for Deetctor for OpenAI. Before the change running quick example ended in validation error
   
   After the change it was working as intended:
   
   But now the system prompt became similar to Anthropic. Not sure whether I changed what should have been changed.

[lbeurerkellner]

Thanks a lot, running the tests to see if the updated prompt still works for OpenAI models

[lbeurerkellner]

I see there are some test failures here, I think we have to make sure client_kwargs is supported everywhere.

[L3G5]

Thanks for the feedback (and for the Invariant: really like it)!
I will try to fix it, doesn't seem too hard.