[identity] [5/n] Create search result message type for identity search

Summary:
This creates a search result type in shared and uses it in the identity search websocket server

Depends on 10848

Test Plan: Successfully tested on local dev. Prefix queries return correct response

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: kamil, ashoat, tomek

Differential Revision: https://phab.comm.dev/D10851