Provide API to list Wi-Fi client devices

Provide API to allow frontend to get a list of devices connected over the Wi-Fi. Both the main Wi-Fi (aka all Wi-Fis whose network is lan) and Guest Wi-Fi (aka all Wi-Fis whose network is guest_turris). We should try to provide some name for each device and display link speed and signal quality.

Useful command for this would be iwinfo.