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.

Built from https://develop.svn.wordpress.org/trunk@51962


git-svn-id: http://core.svn.wordpress.org/trunk@51551 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/widgets-form-blocks.php

@@ -18,7 +18,7 @@ $current_screen->is_block_editor( true );
 $block_editor_context = new WP_Block_Editor_Context();
 
 $preload_paths = array(
-	array( '/wp/v2/media', 'OPTIONS' ),
+	array( rest_get_route_for_post_type_items( 'attachment' ), 'OPTIONS' ),
 	'/wp/v2/sidebars?context=edit&per_page=-1',
 	'/wp/v2/widgets?context=edit&per_page=-1&_embed=about',
 );