[keyserver] Use endpointValidators in keyserve code

Summary:
issue: ENG-8080
Since we need to have endpoint bound to output validators both on lib and keyserver, it would be nice if we could avoid redundant code

Test Plan: checked a couple of endpoints, and verified that no errors are thrown if outputs are validated with validators specified by endpointValidators for those endpoints.

Reviewers: tomek, kamil, bartek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12031