Create module for using Opportunistic Wireless Encryption

[ilario]

Opportunistic Wireless Encryption (OWE) is a kind of encryption of an AP that does not need of a password.
So people can have their traffic encrypted even if there is no authentication.
Clearly, this makes sense only if also the inter-nodes ieee802.11s mesh traffic is encrypted.

The documentation of its usage in OpenWrt is scarce:

• https://openwrt.org/docs/guide-user/network/wifi/basic#common_options1
• https://forum.openwrt.org/t/owe-and-transition-ssid/43502/2
• openwrt/openwrt@574539e

Very likely, the roaming that we have now between open APs would not work.
So this should be activated only on the apname interfaces, for which the roaming does not happen.

[ilario]

Doubt: if there is encryption, will the roaming work?
Would we need something like 802.11r (discussed in #253)?