Assert that an added introducer has certification capabilities

src/commands/introducer/add.rs

@@ -45,6 +45,7 @@ pub async fn dispatch(cmd: IntroducerAddCommand, config: HuskConfigContainer) ->
             .filter_map(|c|
                 c.with_policy(policy, None).ok()
             )
+            .filter(|vc| crypto::can_certify(vc))
             .collect();
 
         if let Some(cert) = certs.first() {