feat(external-gateway): add gateway client domain and DNS record support for dcrouter integration

ts/opsserver/handlers/domains.handler.ts

@@ -97,5 +97,16 @@ export class DomainsHandler {
         },
       ),
     );
+
+    this.typedrouter.addTypedHandler(
+      new plugins.typedrequest.TypedHandler<interfaces.requests.IReq_GetGatewayDomains>(
+        'getGatewayDomains',
+        async (dataArg) => {
+          await requireAdminIdentity(this.opsServerRef.adminHandler, dataArg);
+          const domains = await this.opsServerRef.oneboxRef.externalGateway.getGatewayDomains();
+          return { domains };
+        },
+      ),
+    );
   }
 }