Query: Add a search_columns argument to control which fields are searched in a search query.

Previously, the `s` argument of the `WP_Query::parse_query()` method searched the `post_title`, `post_excerpt`, and `post_content` fields, with no way of controlling this apart from using the `posts_search` filter and adjusting the SQL manually. This changeset adds the ability to specify which fields are searched when performing a query, using the `search_columns` argument.

Props johnbillion, birgire, petitphp, audrasjb, costdev, mukesh27.
Fixes #43867.


git-svn-id: https://develop.svn.wordpress.org/trunk@55248 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -249,6 +249,7 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 			'parent'         => 'post_parent__in',
 			'parent_exclude' => 'post_parent__not_in',
 			'search'         => 's',
+			'search_columns' => 'search_columns',
 			'slug'           => 'post_name__in',
 			'status'         => 'post_status',
 		);
@@ -2891,6 +2892,16 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 			);
 		}
 
+		$query_params['search_columns'] = array(
+			'default'     => array(),
+			'description' => __( 'Array of column names to be searched.' ),
+			'type'        => 'array',
+			'items'       => array(
+				'enum' => array( 'post_title', 'post_content', 'post_excerpt' ),
+				'type' => 'string',
+			),
+		);
+
 		$query_params['slug'] = array(
 			'description' => __( 'Limit result set to posts with one or more specific slugs.' ),
 			'type'        => 'array',