Add constraints to image.settings and mark it fully validatable

Closes #3425870

core/lib/Drupal/Core/Validation/ConstraintManager.php

@@ -12,6 +12,7 @@
 use Symfony\Component\Validator\Constraints\Blank;
 use Symfony\Component\Validator\Constraints\Callback;
 use Symfony\Component\Validator\Constraints\Choice;
+use Symfony\Component\Validator\Constraints\Image;
 use Symfony\Component\Validator\Constraints\NotBlank;
 
 /**
@@ -116,6 +117,11 @@ public function registerDefinitions() {
       'class' => Choice::class,
       'type' => FALSE,
     ]);
+    $this->getDiscovery()->setDefinition('Image', [
+      'label' => new TranslatableMarkup('Image'),
+      'class' => Image::class,
+      'type' => ['string'],
+    ]);
   }
 
   /**