[review] USSD support for MBIM

[Aleksander Morgado]

Hey,

This MR adds USSD support for MBIM devices, and while at it, also
ported the original MMBroadbandModem USSD support to GTask:

https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_requests/23

The MBIM USSD logic basically matches what we had for the AT+CUSD
based logic. The main difference being that the actual strings are
always encoded (GSM7 or UCS2) in the MBIM USSD logic while the strings
passed to AT+CUSD were all in the current modem charset, except for
Huawei modems (which are also always in GSM7/hex).

Comments?

-- 
Aleksander
https://aleksander.es