Add interface for abstract OpenAI-equivalent API

[caufieldjh]

From #70:

Currently, I cannot use ontoGPT in my organization as it is due to security concerns around external APIs like OpenAI's. To address this issue, I suggest exploring the possibility of using open-source, local LLMs and their APIs as an alternative to OpenAI's API.

I have a similar use case, however, could it ever be abstracted even further, with an option to use in-house LLM services? We are currently working on such a ChatGPT alternative, and it would be amazing to simply plug in that API for ontoGPT...

Originally posted by @remerjohnson in https://github.com/monarch-initiative/ontogpt/issues/70#issuecomment-1604285035