[lib] Make stringForUser and useStringForUser work with any user types

Summary:
This change makes it so UserInfo, CurrentUserInfo, AccountUserInfo, etc. all work with these functions.

This allows us to use a single entrypoint for ENS-fetching across a variety of different contexts.

Depends on D6419

Test Plan: This is a types-only change, so I tested with Flow

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6420