Provide common way to specify cert overrides

[jaycle]

Thanks for such a cool and useful tool! I'm not sure if this is in scope of the project, but I just started using llm with gemini on a company computer that has self-signed certificates. I had to dig into the implementation of llm-gemini to see that it was using httpx, then figure out how to override the SSL chain for that library.

It might be too niche of a use-case, but I could see value in having a more global override, since I believe some plugins may need a different way to specify the trusted cert, depending on implementation.

References:
https://www.python-httpx.org/environment_variables/