Add a non-hook way to get the current authed user

[kmjennison]

Is your feature request related to a problem? Please describe.

Developers might want to get the authenticated user outside of a React component context. For example, an app we're building has a fetcher—used within a Relay environment that's created outside of the React tree—that needs the user's ID token. It's been unnecessarily difficult to get access to the latest AuthUser instance in this context. Our app's state management is partly to blame for the complexity, but our lives would be easier if we could just call for the latest ID token directly without relying on a React hook. For now, we ended up getting the current user directly from Firebase when on the client side (PR).

Describe the solution you'd like and how you'd implement it

This library could provide the equivalent to Firebase's firebase.auth().currentUser, returning an AuthUser instance. It would also need to be functional on the server side.

Is this a breaking change?

No.

Describe alternatives you've considered

None that I can think of. Alternative suggestions welcome!