feat(dns): allow setting an externally managed cluster domain

pkg/client/deploy/deploy.go

@@ -306,8 +306,8 @@ func (d *Deployment) Plan(ctx context.Context) (ServicePlan, error) {
 		return ServicePlan{}, fmt.Errorf("get cluster domain: %w", err)
 	}
 	specResolver := &ServiceSpecResolver{
-		// If the domain is not found (not reserved), an empty domain is used for the resolver.
-		ClusterDomain: clusterDomain,
+		// If no domain is configured, an empty domain is used for the resolver.
+		ClusterDomain: clusterDomain.Name,
 	}
 
 	resolvedSpec, err := specResolver.Resolve(d.Spec)