Allow pingBridge endpoint for all users with access to the plugin admin

We call the endpoint on the admin pages and the restricted access was causing
that editor users were getting false error notices.
[MAILPOET-6189]

mailpoet/lib/API/JSON/v1/Services.php

@@ -57,6 +57,7 @@ class Services extends APIEndpoint {
 
   public $permissions = [
     'global' => AccessControl::PERMISSION_MANAGE_SETTINGS,
+    'methods' => ['pingBridge' => AccessControl::PERMISSION_ACCESS_PLUGIN_ADMIN],
   ];
 
   public function __construct(