[feature] Monitor and display WiFi session hostnames

[c-gabri]

Is your feature request related to a problem? Please describe.
It is difficult to identify WiFi clients only by their MAC addresses, especially when there are many of them.

Describe the solution you'd like
If the hostname associated to a MAC address is know to an AP (e.g. because it runs a DHCP server or has its /etc/ethers regularly populated by a DHCP server), the AP should send this information to the OpenWisp server, which should display it as an additional field in the corresponding WiFi session.