[lib] Add UserInfosHandler

Summary:
issue: ENG-7743
Add a scaffolding of a handler component that will be used to fetch usernames and other user infos for entriens in the user store that only have an id.
It is unusual to be adding such empty diffs, but @kamil will need this handler in his work, so I am creating it now to avoid conflicts

Test Plan: Rendered this handler in App, manually added user infos {id: '1000'}, {id: '5000'}. Logged userInfosWithMissingUsernames in the useEffect and checked that the two user infos were present, and only them.

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, kamil

Differential Revision: https://phab.comm.dev/D11932