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'?>
<OPCUAClient xmlns:xsd="http://www.w3.org/2001/XMLSchema" Server_URI="opc.tcp://127.0.0.1:4840/freeopcua/server/">
  <AuthType>
    <x509 Certificate="my_cert.der" PrivateKey="my_private_key.pem">
      <Policy/>
      <Mode/>
    </x509>
  </AuthType>
</OPCUAClient>