fix(ocpp2.0): add proper response handlers for certificate commands + handler coverag...

fix(ocpp2.0): add proper response handlers for certificate commands + handler coverage tests

Replace emptyResponseHandler with proper logging handlers for
Get15118EVCertificate, GetCertificateStatus, and SignCertificate
responses. Add handler registration coverage tests for both OCPP 1.6
and 2.0: every RequestCommand must have a response handler, every
IncomingRequestCommand must have an incoming request handler.