Add moderation models

[sashabaranov]

There's are a bunch of new Moderation models available now, let's add them to our constants

https://platform.openai.com/docs/models/overview

[ealvar3z]

@sashabaranov Do you want this in completion.go or on a separate file? I ask because CreateCompletion() defaults to "/completions" not the "/moderations" urlSuffix on the completion request? Please advise.

[ealvar3z]

@sashabaranov never mind, this is already implemented in moderation.go. I think this issue should be closed then.