Introduce an uuid for a port (PON/NNI/UNI) in similar lines as the device id

Currently a port (device.proto, message Port) can be uniquely identified using the combination of the port_num and the device_id.

If would be easier and simpler for NB systems if the port is identified with a uuid similar to how a device is identified. If this uuid is present in the KPIs/Events and the vCore interface where Ports are reported, then NB systems can easily correlate the information source.

Propose that we introduce an field of type uuid in the Port structure and report this in the events and KPIs for the ports.