Complete CriticalEvent Logic

[VigneshVSV]

The idea of the CriticalEvent object was that one can receive acknowledgement for the server sent event. There is a similar idea in the thing description to support dataResponse for events.

Possible steps involved

• solidify the CriticalEvent concept based on the suggestion given by the Thing Description spec. This may even involve renaming CriticalEvent to something else to indicate an arbitrary response apart from a simple acknowledgement is also supported.
• Create a message in ZMQ & modify EventLoop.execute_once which will call the CriticalEvent's data response accepting method
• Define how errors will be raised to the user
• Idea should work for both HTTP & ZMQ transport (the two that we support now) / protocol agnostic

Please fork main-next-release branch and merge it there.