feat(llms): support vLLM's OpenAI-compatible server (#9179)
This PR aims at supporting [vLLM's OpenAI-compatible server
feature](https://vllm.readthedocs.io/en/latest/getting_started/quickstart.html#openai-compatible-server),
i.e. allowing to call vLLM's LLMs like if they were OpenAI's.
I've also udpated the related notebook providing an example usage. At
the moment, vLLM only supports the `Completion` API.
