Add an API to the BDX transfer session so that a caller can get the next output event generated by the TransferSession state machine without polling.

[nivi-apple]

Currently, there is a PollOutput API which requires the caller to poll frequently to get the next output event generated by the TransferSession and the caller needs to wait up to the polling interval to get the next output event which makes the BDX transfer slower. We would like to add an asynchronous transfer facilitator that can use an event driven approach to drive the BDX transfer and be able to get the next output event on demand rather than waiting on a polling based mechanism.