[feature] Allow updating only the WiFi client list

[nemesifier]

Related to openwisp/openwrt-openwisp-monitoring#163.

We need to allow updating the list of wifi clients, not sure if we need to add a separate REST API endpoint for this or whether it's better to allow the current REST API endpoint to handle this specific case.

More discussion is needed.

[rubenbe]

IMHO it would be best to add another payload type (next to DeviceMonitoring) on the same endpoint.
That would be the most flexible solution, which in theory could also allow 3rd parties (like me) to add more types for custom monitoring.

PS I'm not related to the OpenWISP project, but I'm currently developing OpenSOHO (an OpenWISP for < 20 devices) which uses the awesome OpenWISP daemons on OpenWRT. Thanks for the amazing software and feel free to ignore my suggestion ;)

https://github.com/rubenbe/opensoho