define a prft related event

[RufaelDev]

While not strictly events, receiving prft box might also trigger callbacks using event API, it would be nice to have a scheme defined for receiving these in order to ease testing of low latency setups

[haudiobe]

(Event TF 2019/06/14): Generally supportive, but needs a definition.

• Define an event scheme to subscribe to it, does the @value map to the flag of the prft
• Define the processing mechanism
• Define what the payload of the event will be. Typically it will be the NTP timestamp.
• Needs extension to processing model as prft is not carried in emsg
• Before doing so, we need to agree that this is a valid use case
• Let's verify the use case from the Live case