Qute: introduce template extensions with namespaces

[mkouba]

API:

@TemplateExtension(namespace = "str") 
class StringUtils {
 
  String format(String fmt, String... args) {
      return String.format(fmt, args);
   }

   static String reverse(String val) {
      return new StringBuilder(val).reverse().toString();
   }
}

and the usage:

{str:format('Value is %s', foo)}
{str:reverse('hello')}

It's like "calling static methods from templates".