refactor(aggregator): add support for operational certificate without cold verification key

Author: jpraynaud

mithril-aggregator/src/services/signer_registration/verifier.rs

@@ -48,7 +48,7 @@ impl SignerRegistrationVerifier for MithrilSignerRegistrationVerifier {
                     .get_current_kes_period()
                     .await?
                     .unwrap_or_default()
-                    - operational_certificate.start_kes_period as KesPeriod,
+                    - operational_certificate.get_start_kes_period() as KesPeriod,
             ),
             None => None,
         };