[identity] Create module for HTTP handler

Summary:
Part of ENG-8289.
Moved HTTP handler to a separate module. Websocket server delegates to it when given request is not a "websocket upgrade" request.

Test Plan: Used Postman to GET http://localhost:51004/health - response returns "OK". Other non-search endpoints return 404

Reviewers: will, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12301