Add support to LangServe with FastAPI

[avelino]

https://python.langchain.com/docs/langserve

This library is integrated with FastAPI and uses pydantic for data validation.

talkd/dialog uses LangChain and we probably won't switch frameworks. Given this statement, it's important to follow the evolution of LangChain and the projects they maintain to keep up to date.

warning: I know that LangChain is a new project and will have updates that break our implementations. That's fine, it's part of our choice to use LangChain - we'd have this kind of problem using another framework.

proposal

With the above context in mind, it makes sense to swap the FastAPI us for the LangServer abstraction (which uses FastAPI underneath).

premise

we can't break the signatures of the current endpoints we can have other endpoints and warn those who are using the old one to switch to the new one, but at this point we can't break what works