e2ee: advertise e2ee support in caps

JitsiConference.js

             if (features.has('http://jitsi.org/protocol/jigasi')) {
                 participant.setProperty('features_jigasi', true);
             }
+
+            if (features.has('https://jitsi.org/meet/e2ee')) {
+                participant.setProperty('features_e2ee', true);
+            }
         })
         .catch(() => false);
 };

modules/xmpp/xmpp.js

         if (this.connection.rayo) {
             this.caps.addFeature('urn:xmpp:rayo:client:1');
         }
+
+        if (browser.supportsInsertableStreams()) {
+            this.caps.addFeature('https://jitsi.org/meet/e2ee');
+        }
     }
 
     /**