Allow users to define their own "provider" by decoupling the API style from predefined Provider types

[nvms]

@capdevc had the idea to decouple the API style from the provider type here:

https://github.com/nvms/wingman/pull/5#issuecomment-1586367918](https://github.com/nvms/wingman/pull/5#issuecomment-1586367918)

If solutions like https://github.com/go-skynet/LocalAI become more commonplace, this change could become more valuable to users that want this extension to use their own self-hosted solution that intentionally replicates the API of some bigger player in the space such that tools like wingman can more easily integrate with them.