REST API: Add URL Details endpoint.

Adds a new REST API endpoint (`/wp-block-editor/v1/url-details`) for retrieving information from an external URL.

Information retrieved:

* Title: content of the `<title>` element
* Icon: favicon image link
* Description: content of the `description` or `og:description` meta element
* Image: OG image link

This endpoint is used by the block editor for link previews.

Props get_dave, aduth, andraganescu, beaulebens, hellofromTonya, kevin940726, mamaduka, marekhrabe, mnelson4, noisysocks, obenland, ocean90, retrofox, shaunandrews, spacedmonkey, swissspidy, timothyblynjacobs, xknown, youknowriad.
Fixes #54358.
Built from https://develop.svn.wordpress.org/trunk@51973


git-svn-id: http://core.svn.wordpress.org/trunk@51562 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api.php

@@ -337,6 +337,10 @@ function create_initial_rest_routes() {
 	$site_health = WP_Site_Health::get_instance();
 	$controller  = new WP_REST_Site_Health_Controller( $site_health );
 	$controller->register_routes();
+
+	// URL Details.
+	$controller = new WP_REST_URL_Details_Controller();
+	$controller->register_routes();
 }
 
 /**