code: @SuppressWarnings("unchecked")
      public List<Class<Object>> getExtensionsOrdered(final Class<? extends FeatureInstaller> installer) {
          final List<Class<?>> extensions = holder.getExtensions(installer);
          return extensions == null ? Collections.emptyList() : (List) extensions;
      }
docs: Returned installers are ordered by execution order according to
  {@link ru.vyarus.dropwizard.guice.module.installer.order.Order} annotation.
  Note: not all installers supports ordering (in any case returned order is the same order as
  extensions were processed by installer).
  
  @param installer installer type
  @return ordered list of extensions installed by provided installer or empty list