refactor(auth): use generated IAM client instead of handwritten interactions with the service #4335
Description
Similar to #4334, we have multiple interactions with the IAM Service in the auth crate, but so far we have been handwriting those interactions with the reqwest create and missing nice things like built in retry and type safety.
One issue is that all crates depend on auth, so there is a circular dependency problem here to be solved. @coryan gave some ideas to circumvent this that I don't recall anymore.