New parameters: gateway.id and device.model, reported on asset and anchor messages.
gateway.id carries the identifier of the gateway that relayed the message. device.model reports the node's role in the mesh, either asset or anchor, derived from the type of measurement the node sends.
device.model can be used in a webhook to create devices automatically with the correct device type: the value matches the device type name, so a webhook reacting to traffic from an unregistered node can map it directly without any translation. Nodes whose role cannot be determined from the message do not report the parameter at all, so the absence of device.model is itself the signal to leave the device type alone.
