Responder Flow
ResponderFlow is a Flow that is started by receiving a message from a peer.
A ResponderFlow must be annotated with InitiatedBy to be invoked by a session message. If both these requirements are met, then the flow will be invoked via ResponderFlow.call which takes a FlowSession. This session is created by the platform and communicates with the party that initiated the session.
Flows implementing this interface must have a no-arg constructor. The flow invocation will fail if this constructor does not exist.