Build config option for disabling "wpa_supplicant" support and changing the default "wifi.backend".

It would be handy if there were a meson/autoconf build configuration option that can be used to disable support for the wpa_supplicant wifi backend in distros such as buildroot that allow NetworkManager to be built/used without wpa_supplicant.

It would also be handy if there were an option to set the default wifi.backend to iwd at compile time so that NetworkManager can use that backend automatically in cases where it is known that wpa_supplicant will not be available. It may also be a good idea to have NetworkManager automatically attempt to use iwd at runtime if wpa_supplicant is not available.