Add list models function in BaseLLMClient

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We can add and abstract method to BaseLLMClient like:
def list_models(**kwargs) -> list[str]
And then we need to implement it in the different clients, we can add a static constants that will return the LLMs names or we can make a call to the relative urls.
Here is OpenAI docs for this kind of endpoint: https://platform.openai.com/docs/api-reference/models/list