feat: add langchain support

[dexhunter]

• Add langchain backend (Upcoming Features and Future Directions of Chat Arena #36)

Note: Currently it's just for openai, I think for general llm need to make the backend base more dynamic or conform to some settings by langchain framework. We can follow this example which utilizes from langchain.schema and langchain.output_parsers but not sure if it's desired since we might have less freedom to define some prompts etc but in terms of programming it's less work since we will have some out-of-box functions.

[elliottower]

Looks good to me, would be nice if there were an easy way to plug in any custom stuff defined in langchain, but this example can clearly be extended. I think another key feature would be langchain agents, either making a wrapper for them to work as a backend or making a custom agent class that can be extended.

[dexhunter]

either making a wrapper for them to work as a backend or making a custom agent class that can be extended.

Either way is fine with me, let me know which way you guys decide to go and I can implement accordingly. cc @yuxiang-wu

[yuxiang-wu]

Thank you for your contribution @dexhunter ! I think putting it as a backend is good for now. We can work on a more complex agent later. Thank you @elliottower for the discussion too!