[identity] [6/n] Requires a Search Query json message from client for user identity search

Summary:
This requires the client to send a Search Query json object when using identity search. This makes it clear the user search type and query (i.e. prefix vs exact).

As an example, after authenticating with the websocket server, to prefix search, you would send:

{"prefix": "..."}

Depends on D10851

Test Plan: tested locally after authenticating.

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10873