Support querying/enabling/disabling call waiting network service
As per 3GPP TS 22.083, e.g.:
$ sudo mmcli -m 0 --voice-query-call-waiting
call waiting service is disabled
$ sudo mmcli -m 0 --voice-enable-call-waiting
operation successful
$ sudo mmcli -m 0 --voice-query-call-waiting
call waiting service is enabled
$ sudo mmcli -m 0 --voice-disable-call-waiting
operation successful
$ sudo mmcli -m 0 --voice-query-call-waiting
call waiting service is disabled