Support setting 4addr (WDS) mode per WiFi connection

Under Linux, a Wi-Fi interface cannot be enslaved to a bridge interface, unless
4addr mode (a.k.a. WDS mode) is set on the interface. However, many access
points, prohibit (for security reasons) or don't support 4addr frames. This new
property allows NewtorkManager to enable 4addr mode before enslaving an
interface to a bridge and possibly disabling it when switching to another
56 jobs for kn/4addr-mode in 81 minutes and 45 seconds (queued for 4 seconds)