C++ offboard example: change publishers QoS profile and timer frequency

[marco-bonazza]

• Specified the QoS profiles in all publishers
• Modified timer frequency to be the same as the one reported in https://docs.px4.io/main/en/flight_stack/controller_diagrams.html for position control
• Modified publish_vehicle_command function for better visibility of the specific settings needed to engage offboard mode in the new function engage_offBoard_mode
• Added land function (land) to conclude the demo

Also there is no more need to go into offboard mode through QGC and then start the node

Signed-off-by: Marco Concetto Bonazza marco98.bonazza@yahoo.it

[ViniciusAbrao]

I have translated the code to Python including: arm, take off, offboard flight and land:

https://github.com/ViniciusAbrao/px4_ros2_xrcedds/blob/master/offboard_control.py

[hamishwillee]

@mrpollo Can we get this tested/merged too?