[web] introduce useUserRelationshipUserInfos

Summary:
PLEASE NOTE THAT THIS DIFF AND SUBSEQUENT DIFFS IN THIS STACK WILL NOT BE LANDED UNTIL MORE OF THE REDESIGN IS READY SINCE THIS WILL CAUSE REGRESSIONS IN PROD

In order to use the add users list for both add friends + add members, we need to make this component completely generic. Right now there is a lot of business logic for searching for any user to add as a friend that needs to be factored out.

As a first step to this I introduced useUserRelationshipUserInfos and moved all the search logic from AddUsersList into this new hook

Depends on D10849

Test Plan: flow + confirmed the add friends modal continues to work as expected

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10936