REST API: Support meta registration for specific object subtypes.

Introduce an `object_subtype` argument to the args array for `register_meta()` which can be used to limit meta registration to a single subtype (e.g. a custom post type or taxonomy, vs all posts or taxonomies).

Introduce `register_post_meta()` and `register_term_meta()` wrapper methods for `register_meta` to provide a convenient interface for the common case of registering meta for a specific taxonomy or post type. These methods work the way plugin developers have often expected `register_meta` to function, and should be used in place of direct `register_meta` where possible.

Props flixos90, tharsheblows, spacedmonkey.
Fixes #38323.



git-svn-id: https://develop.svn.wordpress.org/trunk@43378 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -1989,6 +1989,39 @@ function delete_post_meta_by_key( $post_meta_key ) {
 	return $deleted;
 }
 
+/**
+ * Registers a meta key for posts.
+ *
+ * @since 5.0.0
+ *
+ * @param string $post_type Post type to register a meta key for. Pass an empty string
+ *                          to register the meta key across all existing post types.
+ * @param string $meta_key  The meta key to register.
+ * @param array  $args      Data used to describe the meta key when registered. See
+ *                          {@see register_meta()} for a list of supported arguments.
+ * @return bool True if the meta key was successfully registered, false if not.
+ */
+function register_post_meta( $post_type, $meta_key, array $args ) {
+	$args['object_subtype'] = $post_type;
+
+	return register_meta( 'post', $meta_key, $args );
+}
+
+/**
+ * Unregisters a meta key for posts.
+ *
+ * @since 5.0.0
+ *
+ * @param string $post_type Post type the meta key is currently registered for. Pass
+ *                          an empty string if the meta key is registered across all
+ *                          existing post types.
+ * @param string $meta_key  The meta key to unregister.
+ * @return bool True on success, false if the meta key was not previously registered.
+ */
+function unregister_post_meta( $post_type, $meta_key ) {
+	return unregister_meta_key( 'post', $meta_key, $post_type );
+}
+
 /**
  * Retrieve post meta fields, based on post ID.
  *