[keyserver] Start validating responders output

Summary:
Use the validators and the validateOutput function introduced in the previous diff in the api responders. The id schema conversion is behind the flag for now, but the outputs are still validated.

Depends on D7711

Test Plan: Test web and ios emulator if all outputs are validated correctly and no server error is thrown.

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7712