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/rest-api/fields/class-wp-rest-comment-meta-fields.php

@@ -27,6 +27,17 @@ class WP_REST_Comment_Meta_Fields extends WP_REST_Meta_Fields {
 		return 'comment';
 	}
 
+	/**
+	 * Retrieves the object meta subtype.
+	 *
+	 * @since 5.0.0
+	 *
+	 * @return string 'comment' There are no subtypes.
+	 */
+	protected function get_meta_subtype() {
+		return 'comment';
+	}
+
 	/**
 	 * Retrieves the type for register_rest_field() in the context of comments.
 	 *