Support a "replyWith" in mcrew services

[jsccast]

Currently cmd/mcrew has an internal service for HTTP requests. That service supports "replyTo":MACHINE_ID in a request, which will cause the response to include "to":MACHINE_ID.

As an aid to writing a machine specification that will execute "flows" (more on that later), "replyWith":SOME_DATA in a request should result in the response including "with":SOME_DATA.

[boriwo]

Yes, that's a great idea. In practice, I've been holding such data in the bindings of the requesting sheen, only to merge it with the http response once it comes back later. In a way this would be similar to how timers work, were the message field in a timer request is the equivalent piece of data that replyWith would be for http. Maybe just call it message in both cases? On a related note, I'm trying to assemble a list of all such pieces of semantically loaded meta data that's floating around so we have a reference catalog but also to be able to harmonize the syntax across all components.

[jsccast]

The HTTP situation seems to have a different structure than the timers service. The HTTP response is itself a message, and the service can embed a "to" property based on the "replyTo" property in the request. In contrast, the entire timer message is provided by the timer requester. So "replyWith" seems to fit the HTTP service protocol better than the "message" thing. Will contemplate!