title: Retrieve endpoint data from wp-json inside a add_action
body: <p>I have endpoint which has a page id of <code>2</code>. I would like to retrieve that value <code>2</code> which is the page id inside the WordPress RESTFul API Filter.</p>
  
  <p><strong>Example:</strong></p>
  
  <pre><code>add_action( 'acf/rest_api/page/get_fields', function( $data ) {
      var_dump($data);
  });
  </code></pre>
  
  <p><strong>My Endpoint:</strong>
  <a href="http://www.example.com/wp-json/acf/v3/pages/2/" rel="nofollow noreferrer">http://www.example.com/wp-json/acf/v3/pages/2/</a></p>