Add chat function in BaseLLMClient

[uripeled2]

We can add and abstract method to BaseLLMClient like:
def chat_completion(messages, **kwargs) -> list[str]
We need to implement it in the different clients, we can define a scheme similar to what LangChain did with AIMessage, HumanMessage and SystemMessage
Here is OpenAI docs for this kind of endpoint: https://platform.openai.com/docs/api-reference/chat