docs: @SuppressWarnings("unchecked")
      public static void addExtensionProvider(String elementName, String namespace,
              Object provider) {
          validate(elementName, namespace);
          // First remove existing providers
          String key = removeExtensionProvider(elementName, namespace);
          if (provider instanceof ExtensionElementProvider) {
              extensionProviders.put(key, (ExtensionElementProvider<ExtensionElement>) provider);
          } else {
              throw new IllegalArgumentException("Provider must be a PacketExtensionProvider");
          }
      }