code: public function getConfigTreeBuilder()
      {
          $treeBuilder = new TreeBuilder();
          $rootNode = $treeBuilder->root('ongr_api');
  
          $rootNode
              ->children()
                  ->arrayNode('versions')
                      ->info('Defines api versions.')
                      ->useAttributeAsKey('version')
                      ->prototype('array')
                      ->children()
                          ->scalarNode('versions')
                              ->info('Defines a version for current api endpoints.')
                              ->example('v1')
                              ->end()
                          ->append($this->getEndpointNode())
                          ->end()
                  ->end()
              ->end();
  
          return $treeBuilder;
      }
docs: {@inheritdoc}