[keyserver] Introduce responder validators

Summary: Introduce validators for entry, keys, siwe, message report, relationship and search responders and tests, most of which are taken from real requests that I've logged. I decided to not split it in multiple diffs because it's mostly just 1-2 validators per responder type and each change is localized to the responder.

Test Plan: yarn jest

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7685