AP_DDS: Service to check if vehicle is armable

[tizianofiorenzani]

Issue

Closes #28286

What Changed

• Create a new service /ap/prearm_check of type [std_srvs/srv/Trigger] https://docs.ros.org/en/noetic/api/std_srvs/html/srv/Trigger.html)

Response:

• success (true if armable)
• message: a string message (Vehicle is armable or Vehicle is NOT armable).

Test

Run the SITL and then use the DDS test to verify that vehicle is armable:

colcon build --packages-up-to ardupilot_dds_tests 
source install/setup.bash
ros2 run ardupilot_dds_tests pre_arm_check

Or call the service manually:

ros2 service list

/ap/arm_motors
/ap/mode_switch
/ap/prearm_check

ros2 service call /ap/prearm_check std_srvs/srv/Trigger 
requester: making request: std_srvs.srv.Trigger_Request()

response:
std_srvs.srv.Trigger_Response(success=True, message='Vehicle is Armable')

[tizianofiorenzani]

@Ryanf55 I have trouble getting the services working with the pytest fixture. I have added a runtime test that can be run with

ros2 run ardupilot_dds_tests pre_arm_check