External processor should be able to return binary data as an immediate response

*Description*:
External processor ImmidateResponse.Builder has a method called setBodyBytes however this method checks to make sure the body bytes are UTF-8. We are wanting to leverage external processor to create a response cache. When we have previously cached a response we want to return that response via the ImmediateResponse mechanism. Because immediate response only allows UTF-8, we are limited to caching only text type responses, which makes external processor unsuitable for our usecase.