add mqtt device mapper

[lcw2]

This is an implementation of mapper for mqtt protocol. The aim is to create an application through which users can operate mqtt device and read mqtt device data.

[RyanZhaoXB]

Thanks for this issue. The current mapper-go repository has not implemented a mapper for mqtt device. Do you want to provide
an implementation of mapper for mqtt protocol? You are welcome to create a pull request to merge your code or design.