Support `StreamingOutput` or document alternative in RESTEasy Reactive

Description

I have seen this issue discussed in both #26473 and #15891, but it still remains unclear to me how to do the equivalent of StreamingOutput with RESTEasy Reactive. If StreamingOutput cannot be directly supported as return type for blocking endpoints, I think it would make sense if the documentation provided some clear guidance on how to do this, including how to set headers (e.g. Content-Disposition as in the linked discussion), as RESTEasy Reactive is being promoted as the JAX-RS implementation of choice.

As in the discussion #26473 we also have a blocking endpoint which returns the output of some third-party library and having to first store that as a temporary file is IMO not very nice.

Implementation ideas

No response

[knutwannheden]

I will have to do some more testing, but I think this may do the trick and would allow my endpoints to transparently use StreamingOutput:

@Provider
public class StreamingOutputMessageBodyWriter implements MessageBodyWriter<StreamingOutput> {

    @Override
    public boolean isWriteable(Class<?> aClass, Type type, Annotation[] annotations, MediaType mediaType) {
        return StreamingOutput.class.isAssignableFrom(aClass);
    }

    @Override
    public void writeTo(StreamingOutput blob, Class<?> aClass, Type type, Annotation[] annotations, MediaType mediaType, MultivaluedMap<String, Object> httpHeaders, OutputStream outputStream) throws IOException, WebApplicationException {
        blob.write(outputStream);
    }
}

[Manfred73]

This workaround is a lifesaver! Been troubleshoothing for hours why I got some weird result back from the StreamingOutput (when processing it showed the classname$1… instead of the filecontent). Adding this StreamingOutputMessageBodyWriter does the trick.