Introduce hook_image_derivative_created.

Closes #2940016

core/modules/image/src/Entity/ImageStyle.php

@@ -340,6 +340,13 @@ public function createDerivative($original_uri, $derivative_uri) {
       return FALSE;
     }
 
+    // Inform other modules about the derivative image creation.
+    $this->moduleHandler()->invokeAll('image_derivative_created', [
+      $original_uri,
+      $this->id(),
+      $derivative_uri,
+    ]);
+
     return TRUE;
   }