Support a unified API (such as LiteLLM) for all LLM providers / models

[omri374]

Spacy-LLM currently supports a number of LLM providers, either through huggingface, langchain or REST API. Any thoughts on introducing a unified API provider, such as https://github.com/BerriAI/litellm, to be able to support additional ones? This would reduce the need to maintain a large number of integrations, and would allow users to use additional LLM providers. Happy to work on a PR to this if you think it's a good idea.