REST API: Introduce WP_Post_Type::get_rest_controller() caching method to prevent unnecessary REST controller construction.

Cache REST controller references on their associated post type object to prevent unnecessary controller re-instantiation, which previously caused "rest_prepare_{$post_type}" and "rest_{$post_type}_query" to run twice per request.

Props TimothyBlynJacobs, patrelentlesstechnologycom.
Fixes #45677.


git-svn-id: https://develop.svn.wordpress.org/trunk@46272 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -16,6 +16,14 @@
  */
 class WP_REST_Posts_Controller extends WP_REST_Controller {
 
+	/**
+	 * Instances of post type controllers keyed by post type.
+	 *
+	 * @since 5.3.0
+	 * @var WP_REST_Controller[]
+	 */
+	private static $post_type_controllers = array();
+
 	/**
 	 * Post type.
 	 *