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

<?xml version='1.0' encoding='utf-8'?>
<MQTTClient xmlns:xsd="http://www.w3.org/2001/XMLSchema" Broker_URI="wss://192.168.19.25:18885">
  <AuthType>
    <x509 Client_certificate="" Broker_certificate="test-root-ca.crt" Verify_hostname="false"/>
  </AuthType>
</MQTTClient>