Allow blocking gateway-to-gateway requests

[RafaelAPB]

In certain phases of SATP, gateways negotiate (Stage 0). The client applications may want to continue the process (continue) or cancel it (cancel) based on custom business logic. Provide support for gateways to wait (up to a certain timeout) for client application input in the relevant phases (identifying them is part of the task).

Extend gateway client SDK/BOL to accommodate this change. A possible implementation is the client listening for events that are fired upon the gateway reaching a step where input is needed; the client application can then issue a request accordingly.

cc @AndreAugusto11