There is already the hostapd_stations exporter, which uses
hostapd-utils (more precisely hostapd-cli) to get client statistics.
However, the ubus interface is permanently integrated under hostapd
in OpenWrt. So this exporter needs one dependency less.

For now it exports mainly the rrm statistics. Many people are
interested in what your device supports. The exporter provides
information about the radio-resource-managment extensions.

Signed-off-by: Nick Hainke <vincent@systemli.org>