[lib] Add useFindUserIdentities

Summary:
issue: ENG-7701
This will be used by UserInfosHandler to fetch usernames for new user ids and update proper fields in the user store

Test Plan: Tested with the rest of the stack - Tested that this action can be used to fetch usernames and inser them into the user store

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12217