Refactor namespaced storage pattern into a unified, well-tested class, and document well-known namespaces as constants #192

In several places in our code (OAuthFlow, Authorization, etc.), we use a pattern of storage reads/writes/deletes where we take the incoming key and prefix it with a specific value for isolation. Ideally, we could implement a class that does exactly this key processing so that we do not have to reimplement this pattern every time.

In addition, it would be best if we documented which storage namespaces are used in a centralized place to improve maintainability.