Add 2 functions for Telit and Quectel modems:
  - disableVoiceSupport: disable voice support (IMS and CSFB) and enable SMS only registration flag.
  - getVoiceSupport: get voice support configuration for the current radio.

[GP-1111] mPower R. Apr 2021: +CEMODE shall be set to CEMODE=2

See merge request !40

Switched to the separate fields for SIM MCC and SIM MNC
after a code review.

Fixed the `CellularRadio::getCarrier` implementation to use MCC/MNC
combination from the SIM intead of the serving cell.

Historically the implementation used MCC/MNC data from the cell that
is currently serving the device and provides the registration. This
MCC/MNC code is different from those of the home network in case of
a roaming - the network/carrier that provided the SIM is different
from the one who serves the device at the moment.

The more correct way to determine home carrier for GSM/UMTS/LTE devices
is to read first 5 or 6 digits of the SIM IMSI than encode the carrier.
This commit updates the implementation to use SIM IMSI as a source of
truth for the home network/carrier code.

Extended the implementation of "getSimCarrierCode" to detect the carrier
(home network) based on the MCC/MNC combination.

Historically mPower used ICCIDs to detect the carrier and apply the
carrier-specific settings. ICCIDs are known for Verizon and AT&T, but
the list of known ICCIDs is not exhaustive.

This change allows the firmware to detect more AT&T SIM cards by using
the MCC/MNC combinations (PLMN IDs) when ICCID is not recognized.

Limitation: The SIM shall be relatively new and report its MNC length,
otherwise MCC/MNC can't be determined and so the carrier code.

Added an ability to read the PLMN ID (MCC/MNC combination) of the home
carrier from the SIM.

SIM PLMN ID is a part of the IMSI that contains identifier of the home
network. PLMN ID in turn consists of 3-digit MCC (country code) and
2-or-3-digit MNC (network code). The length of the MNC is stored in the
SIM Elementary File called "Administrative Data" (EFad for short).

The purpose of the new functions is to extract PLMN ID from the SIM
if this information is available.

Added three additional MCC/MNC combinations for the FirstNet networks.

Those combinations were discovered on a 5SG1 radio in the output of
`AT#FWPLS=0,0`.

Added the 'Carrier Code' field to the MCC/MNC table. The new field is
supposed to be a constant Multitech-specific indentifier that is constant
between the firmware versions and does not necessarily depends on a
carrier name.

In particular the new field is needed to detect when there is an AT&T
SIM card inserted and 'UE mode of operation' shall be set to CS/PS mode 2.

Added one lost MCC/MNC entry for the FirstNet network (PLMN ID 313100)
and removed a duplicate MCC/MNC entry for PLMN ID 344930.