@ResultType: new annotation for source code generator

[jknack]

Hints source code generator (jooby annotation processor) to map/adapt a specific return type to use a custom
handler. This annotation if only for source code generator process so only applies for MVC routes.
Example:

   class MyController {
     @GET("/")
     public MySpecialType hello() {}
   }
 }

• Write a code generator:

 @ResultType(types = MySpecialType.class, handler = "customMapping")
 class MySpecialTypeGenerator {

      public static Route.Handler customMapping(Route.Handler handler) {
          return myHandler.then(handler);
      }
  }

• Let jooby annotation processor to know about your handler by setting the jooby.handler annotation processor option:

  jooby.handler=mypackage.MySpecialTypeGenerator

• Generates:

     app.get("/", customMapping(this::hello));

This new annotation replaces the existing ResultHandler supports which works at runtime, while this one works at compile time.

see #3422