Support for envoy's direct_response?

Envoy supports giving a constant http response (“direct_response”,, but it seems this functionality is not exposed in istio.
I tried replacing spec.http.route with spec.http.direct_response, but this is rejected by istio.
In general, how is the relationship of the envoy version to the istio one?

In general, it is not Istio’s goal to expose every possible configuration parameter of Envoy, but rather to develop a powerful and useful API that serves the majority of use cases.

If this is a feature request, can you tell us a little bit about your use case: not just that you want a particular feature, but what problem you are trying to solve and why you think using Envoy in this way is a good solution for you.