get_bearer_token and refresh should return the parsed JSON response.

Currently these only set the values in the json response as attributes of the AuthClient this makes it difficult to know which attributes will be available when, for instance, `get_bearer_token` sets `realm_id` but `refresh` doesn't.

We want results of talking to AuthClient to be able to participate in our strict runtime type checking which involves converting a primitive python type into an [attrs](https://www.attrs.org/en/stable/) dataclass. Having to construct this dictionary out of the AuthClient before we can do that is annoying and error prone.