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

*.pyc
**/__pycache__
.vscode
**/build/**
*.class
**/.svghmithumbs/**
**/my_*.der
**/my_*.pem
tests/tools/Docker/requirements.txt
**/management.json
**/.secret

doc/_build/**
doc/locale/**

C_runtime/**/*.d
C_runtime/**/*.o
beremiz_runtime
**/tmp/**