[azopenai] Model renames

[richardpark-msft]

We have a few models that come from OpenAI that don't match our conventions in Azure. This is renaming/rearranging will affect all of the SDKs, so it's probably post beta.2.

A new nits here (can be fixed later).
ImageGenerationOptions needs a better name, maybe something like ImageGenerationSettings (or perhaps CreateImageSettings so it aligns with CreateImageOptions).
And ideally, it would contain all required settings and the optional ones moved to CreateImageOptions.

Originally posted by @jhendrixMSFT in #21054 (comment)

[jhendrixMSFT]

Thinking more, our typical pattern is that all required params are discrete and only the optional ones are grouped.

[richardpark-msft]

Leaving this as-is for now, we're trying to do less changes from the default client at this point.