MqttTcpTransport.cpp

LDT-5291: Cloud Communication - SW - 01 - Device-RT to Cloud - Integration - Demo Focus - R&I - 03: DEV - Feature Implementation & Code Document [& Diagnostics]
- Removed MqttPublisher, MqttTcpTransport and MqttTransport (to use QtMqtt instead).
- Added MqttClient (wrapping QMqttClient) for MQTT connection to DCS.
- Moved MqttPacket from Comms to tools/DCSsim since it is no longer needed by CloudConnect.
- Updated Comms build files to add the Qt Mqtt module dependency.
- Updated CloudConnectController to use MqttClient.
- Updated CloudConnect.ini to add TLS keys.
- Added SslTcpServer to DCSsim so the sim can accept TLS connections.
- Updated the C++ standard to c++17 in .qmake.conf, all .pro files and all CMakeLists.txt.
- Renamed codegen.pri to codegen_tools.pri.