Add model options for ChatGPT plus

[publicqi]

ChatGPT Plus users can use text-davinci-002-render-paid and text-davinci-002-render-sha models. I changed ChatGPT models to be a list of model name (same as OpenAI api). On the API side, you'll also need to change that to a list of model names.

I thought about fetching available model names from ChatGPT directly, but that seems to break the current layered design.

[wong2]

Thanks, but I prefer the default approach