Support Claude, Llama2 and other LLMs

[Onurbon]

The current implementation relies on OpenAI's APIs but we would want users to be able to use any LLMs.

We can leverage LangChain for this, but we'll also need to provide users with appropriate options to configure not only the models and the prompts, but also the parser that they want to use to process the LLM's output. This will be particularly important when dealing with models such as Llama2 since they cannot return well-formatted JSON outputs.