Add request hook for HTTP query execution

I am using Jena's QueryEngineHTTP for queries and RemoteUpdateRequest for updates. I would like to send a custom header that will differ on each request. Our database, Allegrograph, allows us to pass down custom headers while querying or updating in order to store that information in access logs. For each SPARQL request we want to send down unique request markers and usernames in order to be able to trace from a UI click to backend calls to queries that were performed in the graph. I did some digging around and it seems like it's not possible. Using default headers when configuring Apache's Http Client wouldn't work in this case. The suggestion [~andy.seaborne] made was to create a request hook in HttpOp where we could add additional headers as necessary.