Consider allowing a client to request information about an exchange via non-empty post body

[dlongley]

When a client is starting an exchange, it should be able to send a signal in a non-empty post body to request information about what action the exchange will perform prior to triggering that action. Some exchanges do not send a VPR and immediately issue one or more VCs -- and issuance is performed immediately in response to an empty post body. It would be nice to have an optional feature to show what will be issued to the client prior to issuance, even though a client or user should have received contextual information about that exchange prior to using it.