new gpt3-turbo and whisper API

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
}
}
```