added support for openrouter

[Guido1Alessandro1Trevisan]

Added support for any llm by subclassing ChatOpenAI.

[pyoneerC]

• maybe check if the key is actually there before trying to use it

• looks like some of the error messages are kind of saying the same thing twice

• might be good to throw in some tests, like what happens if one key is missing or both

• could add a short note in the code saying what that new class is for

basically it's good, just needs a little cleanup and maybe a test or two

[Guido1Alessandro1Trevisan]

@pyoneerC The code now checks the key using os.getenv and returns a single error message (instead of two) if the key is set to None. I added a brief note in the Openrouter class to explain its purpose. I tested the code for cases where one key is missing or both are missing, but I feel like adding a test for a missing key might be overkill. Should I perform the test when instantiating the class or when running the CLI command?