code: public function addMiddleware(...$middleware)
      {
          foreach ($middleware as $mw) {
              if (is_callable($mw)) {
                  $this->app->add($mw);
              } elseif (is_string($mw)) {
                  $class = $mw;
                  $callable = new $class($this->getContainer());
                  $this->app->add($callable);
              } else {
                  throw new \InvalidArgumentException('Unsupported type for the middleware');
              }
          }
  
          return $this;
      }
docs: Add application middleware
  The only requirement: $middleware can be a callable or a string for a class name.
  A $middleware may not be necessarily an instance of Kod\BootstrapSlim\Middleware.
  
  @param array ...$middleware
  @return static
  @throws \InvalidArgumentException