[keyserver] Validate siweNonceResponder

Summary:
https://linear.app/comm/issue/ENG-4085/siwenonceresponsevalidator-doesnt-appear-to-be-used
There was a validator for the response but it wasn't used. It doesn't contain any ids to be converted, but we should be consistent.

Test Plan: Call the endpoint, check if it returns the value correctly

Reviewers: atul, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8387