docs: public static void addProperty(Stanza packet, String name, Object value) {
          JivePropertiesExtension jpe = (JivePropertiesExtension) packet.getExtension(JivePropertiesExtension.NAMESPACE);
          if (jpe == null) {
              jpe = new JivePropertiesExtension();
              packet.addExtension(jpe);
          }
          jpe.setProperty(name, value);
      }