REST API: Support custom namespaces for custom post types.

While a custom post type can define a custom route by using the `rest_base` argument, a namespace of `wp/v2` was assumed. This commit introduces support for a `rest_namespace` argument. 

A new `rest_get_route_for_post_type_items` function has been introduced and the `rest_get_route_for_post` function updated to facilitate getting the correct route for custom post types.

While the WordPress Core Block Editor bootstrap code has been updated to use these API functions, for maximum compatibility sticking with the default `wp/v2` namespace is recommended until the API functions see wider use.

Props spacedmonkey, swissspidy.
Fixes #53656.


git-svn-id: https://develop.svn.wordpress.org/trunk@51962 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -1418,6 +1418,7 @@ function get_post_types( $args = array(), $output = 'names', $operator = 'and' )
  *     @type bool         $show_in_rest          Whether to include the post type in the REST API. Set this to true
  *                                               for the post type to be available in the block editor.
  *     @type string       $rest_base             To change the base URL of REST API route. Default is $post_type.
+ *     @type string       $rest_namespace        To change the namespace URL of REST API route. Default is wp/v2.
  *     @type string       $rest_controller_class REST API controller class name. Default is 'WP_REST_Posts_Controller'.
  *     @type int          $menu_position         The position in the menu order the post type should appear. To work,
  *                                               $show_in_menu must be true. Default null (at the bottom).