MQTT Examples: Create _pub _sub command line clones

Mosquitto (and other implementations) do provide command line tools like mosquitto_pub and mosquitto_sub offering basic usage of the protocol from command line.

We should create similar tools as well, with the benefit that we might be able to hook external testing frameworks to it.

However, we will (most likely) not be able to generate the same output, hence glitches are to be expected.