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'?>
<BeremizRoot URI_location="ERPC://localhost:61131">
  <TargetType/>
  <Libraries Enable_Native_Library="true" Enable_Python_Library="false"/>
</BeremizRoot>