[identity] [8/n] Implement Heartbeats on identity search ws server

Summary:
This implemented heartbeats, setting a timing out after 3 seconds and sending a heartbeat every 3 seconds.

Depends on D10874

Test Plan: Tested on local dev. Without sending a heartbeat or search query at least every 3 seconds, the websocket connection will automatically close. The client also receives a heartbeat every 3 seconds as well.

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10875