This PR updates the casing of "OpenAI" when used in Rust identifiers to
match the [Rust naming
guidelines](https://rust-lang.github.io/api-guidelines/naming.html):

> In `UpperCamelCase`, acronyms and contractions of compound words count
as one word: use `Uuid` rather than `UUID`, `Usize` rather than `USize`
or `Stdin` rather than `StdIn`.

Release Notes:

- N/A