Add Paho MQTT C/C++

This patch adds the C and C++ implementations of Paho MQTT
(both libraries are added because the C++ API depends on the C one).
The libraries are configured, built, and installed as external projects
because of they peculiar dependency; they still reside in the build directory.

CMake required version is increased to 3.15.
Paho MQTT C and C++ needs to be installed in a cross-platform way, which is done
by cmake --install introduced in CMake 3.15.

Since the new modules are built separately, they are not to be included in the
RoSA project. Modules that are to be excluded from the RoSA project are
specified in the new CMake variable ROSA_EXCLUDE_SUBMODULE.

A simple example, mqtt-client, is added that demonstrates the very basic
usage of the Paho MQTT C++ asynchrounous client.