Allow metadata to be captured with requests

[3noch]

servant-reflex has the notion of a "tag" (not to be confused with tagging Events, etc.) where an XHR request can be "tagged" with some meta information that will be given back with the response. The meta information has no role in the XHR interaction itself. It's merely a "snapshot" of some state at the time the request was made. This is helpful to track the value of things at the time of a request, even if those values may have changed since the request was made.

It would be useful to have such a feature here as well. For example, I would use it keep track of the form data that was posted via a websocket request. I don't actually want the websocket to send back the form data. I just need to know what that form data was when I made the request. As long as the request comes back with "Ok" then I know the form data in the "tag" is correct. But I can avoid having to send it back in the websocket.

[3noch]

This may be possible by holding a value on the same event as the WebSocket request event.

[dfordivam]

As a work around, I used a simple holdDyn, but if you are sending multiple requests then the implementation will be tricky..
I know how to do this properly in library... but to make it compile is just hard work...

Actually I tried implementing but realized holdDyn is good enough for me... so will come back to this later