MQTT: add JSON payload corresponding to IEC Structured types

- uses https://github.com/cesanta/frozen
- frozen.c is added as a resulting c file in generat_C (fix toolchain_gcc.py as it was breaking dependency checking)
- structured types are converted to C #define, in turn generating json_scanf and json_printf statements

{
    "ID": "wamptest", 
    "active": true, 
    "protocolOptions": {
        "autoPingInterval": 60, 
        "autoPingTimeout": 20
    }, 
    "realm": "Automation", 
    "url": "ws://127.0.0.1:8888"
}