Sample code for Hello World

[sam-tj]

This code is a sample for String based applications.

[sam-tj]

Would you suggest any other changes for this code?

[Acuadros95]

Just tested on an ESP32 and its looking good!

If we plan to merge this on galactic too, it would be nice to use the micro_ros_utilities package to handle the msg memory. Check this tutorial: https://micro.ros.org/docs/tutorials/advanced/handling_type_memory/