Move custom HTTP headers to config producers

Currently custom HTTP headers are configured on CustomMediaTypeResponder which is nice, but on the other hand would be great if we could move it to controller. Maybe creating something like a CustomHttpHeaderProducer (or whatever name) using the same approach done for CorsConfigProducer. For example (just an example):

    public CustomHttpHeaderConfiguration demoConfig() {
        return HttpConfig.enableCustomHttpSupport()
                .addHeader("Auth-Token","xxxx")
                .addHeader("AG-X", "xxxx")
                .build();
    }