wardmodem: Implement translation from AT commands to wardmodem requests.

ATTransceiver handles AT commands from modemmanager. Interpret these commands as
actions on wardmodem state machines.

Add a configuration file format to specify the AT command to state machine
function call mapping.

BUG=chromium:242402
TEST=Run unit-tests
python at_transceiver_unittest.py

Change-Id: I0d07a70c20170d6d724d2457fbc755e591e197b9
Reviewed-on: https://gerrit.chromium.org/gerrit/59146
Reviewed-by: Thieu Le <thieule@chromium.org>
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
5 files changed