Save user agent on open

[MAILPOET-3735]

lib/Router/Endpoints/Track.php

@@ -98,6 +98,7 @@ class Track {
         'queue' => $data->queue_id, // phpcs:ignore Squiz.NamingConventions.ValidVariableName.MemberNotCamelCaps
       ]);
     }
+    $data->userAgent = $_SERVER['HTTP_USER_AGENT'] ?? null;
     return $this->_validateTrackData($data);
   }