Ability to extend device addressing scheme

[lulf]

At the moment, device aliases are by design only used on ingressing events from devices. However, in a use case for eclipsecon using bluetooth mesh, devices have an additional 'address context' attached, which is used by the gateway to identify devices. This context needs to be part of any communication between the gateway and the application that wants to send commands to the device.

This means that in order to send commands to a device, the end application needs to perform a lookup from it's custom addressing scheme to the drogue cloud address in order to send commands to the device.

The implementation currently uses device aliases for allowing gateways to not needing a lookup when forwarding events from devices to the cloud. However, on the way back, either the end application or the gateway needs to look up the address mapping so that the gateway knows which device to forward the event to.