Support for 3GPP rescan and manual 3GPP network selection

[TheOneWithTheBraid]

Right now, the WWAN support in this project only permits to activate given GSM connections present in Network Manager - as well as to toggle GSM in general.

Apart from that basic support, I'm looking for a way to quickly switch the 3GPP network used by the GSM profile.

Right now, this requires to:

# manually scan networks using ModemManager
mmcli -m any --3gpp-scan

# modify the profile to use the discovered 3GPP network
nmcli connection modify my-gsm gsm.network-id XXXXX

# or revert to automatic selection
nmcli connection modify my-gsm remove gsm.network-id

# reconnect with the selected network
nmcli connection down my-gsm
nmcli connection up my-gsm

[firecat53]

I'm open to PR's. I don't have anything to test this with. Thanks for the interest!