title: Is it possible to suspend a flow such that it can be resumed with an RPC-call?
body: <p>I am trying to implement the following use-case in Corda:
  FlowA has been invoked on PartyA via <code>startFlowDynamic</code>. FlowA creates a partially signed transaction and invokes FlowB on PartyB via <code>sendAndReceive</code>. A human user shall now review and manually approve this transaction. Ideally FlowB should suspend after receiving the transaction. I would like to be able to query for suspended instances of FlowB via RPC, and present those (or rather some representation of the transaction therein) to the user in my UI. Then, after the user actions his approval, I would like to resume FlowB via RPC, which would then sign the transaction and return it to FlowA on PartyA.</p>
  
  <p>I noticed that I can inspect suspended flows to some degree via CordaRPCOps.stateMachineAndUpdates and I read the tutorial on progress tracking, but it doesn't quite suffice for my case. I also read that interacting with people from flows is listed as a future feature, I just wondered if there isn't already some way to accomplish this ?</p>