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/taxonomy.php

@@ -1322,6 +1322,39 @@ function has_term_meta( $term_id ) {
 	return $wpdb->get_results( $wpdb->prepare( "SELECT meta_key, meta_value, meta_id, term_id FROM $wpdb->termmeta WHERE term_id = %d ORDER BY meta_key,meta_id", $term_id ), ARRAY_A );
 }
 
+/**
+ * Registers a meta key for terms.
+ *
+ * @since 5.0.0
+ *
+ * @param string $taxonomy Taxonomy to register a meta key for. Pass an empty string
+ *                         to register the meta key across all existing taxonomies.
+ * @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_term_meta( $taxonomy, $meta_key, array $args ) {
+	$args['object_subtype'] = $taxonomy;
+
+	return register_meta( 'term', $meta_key, $args );
+}
+
+/**
+ * Unregisters a meta key for terms.
+ *
+ * @since 5.0.0
+ *
+ * @param string $taxonomy Taxonomy the meta key is currently registered for. Pass
+ *                         an empty string if the meta key is registered across all
+ *                         existing taxonomies.
+ * @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_term_meta( $taxonomy, $meta_key ) {
+	return unregister_meta_key( 'term', $meta_key, $taxonomy );
+}
+
 /**
  * Determines whether a term exists.
  *