REST API: Add batch support for posts and terms controllers.

This also exposes the value of `allow_batch` in `OPTIONS` requests to a route.

A future commit will add batch support to more resources.

Props spacedmonkey, chrisvanpatten.
See #53063.


git-svn-id: https://develop.svn.wordpress.org/trunk@52068 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php

@@ -16,6 +16,14 @@
  */
 class WP_REST_Attachments_Controller extends WP_REST_Posts_Controller {
 
+	/**
+	 * Whether the controller supports batching.
+	 *
+	 * @since 5.9.0
+	 * @var false
+	 */
+	protected $allow_batch = false;
+
 	/**
 	 * Registers the routes for attachments.
 	 *