Added Caller interface to callback object methods.

[dino-rider]

I had trouble using the module in object oriented code. adding this interface and inheriting from it make it much easier to utilize.

[facorazza]

Is this needed considering that the class definition is 4 rows below?

[dino-rider]

Thanks, probably I just left it there from before. Works fine without it.

[facorazza]

This could be a great addition but I have tested the code and it doesn't seem to be functional nor complete

[dino-rider]

I'll update the example to a better one, for now there is a working example using this callback in my other repository https://github.com/dino-rider/IotDevice_test. Also I was working in PlatformIO, but I'm making an arduino example.