Add WillRespond event

So that custom data can be loaded onto a model before it is serialized.
(Tags extension uses this to load tags onto the forum model.)

src/Api/Actions/SerializeAction.php

@@ -3,6 +3,7 @@
 use Flarum\Api\Request;
 use Flarum\Api\JsonApiRequest;
 use Flarum\Api\JsonApiResponse;
+use Flarum\Api\Events\WillRespond;
 use Tobscure\JsonApi\SerializerInterface;
 use Tobscure\JsonApi\Criteria;
 use Illuminate\Http\Response;
@@ -71,6 +72,8 @@ abstract class SerializeAction extends JsonApiAction
 
         $data = $this->data($request, $response = new JsonApiResponse);
 
+        event(new WillRespond($this, $data, $request, $response));
+
         $serializer = new static::$serializer($request->actor, $request->include, $request->link);
 
         $response->content->setData($this->serialize($serializer, $data));