WIP: Add skeleton for MQTT extension based on part of OPC-UA extension.
For now generated code makes no sense but persistently-configurable-C-generating-extension infrastructure is there.
<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot URI_location="ERPC://localhost:61131">
<TargetType/>
<Libraries Enable_Native_Library="true" Enable_Python_Library="false"/>
</BeremizRoot>