Add Attachments processor class for handling ActivityPub media

.github/changelog/2314-from-description

@@ -0,0 +1,4 @@
+Significance: minor
+Type: added
+
+Add unified attachment processor for handling ActivityPub media imports from both remote URLs and local files, with automatic media block generation and Classic Editor support.

activitypub.php

@@ -69,6 +69,7 @@ function rest_init() {
  */
 function plugin_init() {
 	\add_action( 'init', array( __NAMESPACE__ . '\Activitypub', 'init' ) );
+	\add_action( 'init', array( __NAMESPACE__ . '\Attachments', 'init' ) );
 	\add_action( 'init', array( __NAMESPACE__ . '\Comment', 'init' ) );
 	\add_action( 'init', array( __NAMESPACE__ . '\Dispatcher', 'init' ) );
 	\add_action( 'init', array( __NAMESPACE__ . '\Embed', 'init' ) );