Define + export `ServiceTiers` string literal

[s-banach]

Confirm this is a feature request for the Python library and not the underlying OpenAI API.

• This is a feature request for the Python library

Describe the feature or improvement you're requesting

If I want to annotate service_tier in my code, I have to hard-code a type like Literal["auto", "default", "flex", "scale", "priority", "fast"], and then the hard-coded type annotation breaks if I'm using anything other than the latest version of the sdk.

Additional context

No response

[Mahnoor-Zaffar]

Hey, looks like we need a centralized ServiceTiers type, can I add that??