new gpt3-turbo and whisper API

[Moelf]

https://openai.com/blog/introducing-chatgpt-and-whisper-apis

julia> CC = OpenAI.create_completion("-......................", "gpt-3.5-turbo")
ERROR: request status HTTP/2 404: {
  "error": {
    "message": "This is a chat model and not supported in the v1/completions endpoint. Did you mean to use v1/chat/completions?",
    "type": "invalid_request_error",
    "param": "model",
    "code": null
  }
}

[roryl23]

For Whisper, there's a package recently developed that wraps the C++ model implementation rather than the API if that's a better use case for some people. Obviously we'll still implement the API wrapping here.