REST API: Introduce selective link embedding.

Previously the _embed flag would embed all embeddable links in a response even if only a subset of the links were necessary. Now, a list of link relations can be passed in the _embed parameter to restrict the list of embedded objects.

Props rheinardkorf, adamsilverstein, jnylen0, cklosows, chrisvanpatten, TimothyBlynJacobs.
Fixes #39696.

Built from https://develop.svn.wordpress.org/trunk@47224


git-svn-id: http://core.svn.wordpress.org/trunk@47024 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api.php

@@ -1571,3 +1571,25 @@ function rest_preload_api_request( $memo, $path ) {
 
 	return $memo;
 }
+
+/**
+ * Parses the "_embed" parameter into the list of resources to embed.
+ *
+ * @since 5.4.0
+ *
+ * @param string|array $embed Raw "_embed" parameter value.
+ * @return true|string[] Either true to embed all embeds, or a list of relations to embed.
+ */
+function rest_parse_embed_param( $embed ) {
+	if ( ! $embed || 'true' === $embed || '1' === $embed ) {
+		return true;
+	}
+
+	$rels = wp_parse_list( $embed );
+
+	if ( ! $rels ) {
+		return true;
+	}
+
+	return $rels;
+}