[keyserver] Validate websocket output

Summary:
Adds validators for server socket messages with tests, and enables output validation in the socket implementation.

Depends on D7714

Test Plan: Check if web and ios app work fine, and there aren't any errors.

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7749