[web] introduce add users list provider

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 I was attempting to lift the rest of the add friends specific logic from AddUsersList, I noticed that I needed to drill a ton of props in a bunch of places to handle the state needed for AddUsersList. This seemed a bit like a red flag to me, and I thought it would be a lot cleaner/better if I introduced a new provider to handle the state needed in AddUsersList and any relevant components

Depends on D10936

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/D10937