GetAvailableAttributesEvent: add addAttribute()

The common case is that users want to add new attributes, so make
this case easier.

src/Authorization/Event/GetAvailableAttributesEvent.php

@@ -41,4 +41,9 @@ class GetAvailableAttributesEvent extends Event
     {
         $this->attributes = $attributes;
     }
+
+    public function addAttribute(string $name): void
+    {
+        $this->attributes[] = $name;
+    }
 }