AP_DDS: Status Message

[tizianofiorenzani]

• Issue: AP_DDS: Provide vehicle type and mode #28301
• Creates a new /ap/status message
• Publishes the topic only when the status changes.
• The status reports a flag external_control, which will be filled by or after AP_DDS: External Control enable #28429.

Test

• Launch the SITL.
• Open a new terminal and echo the new topic.
• Verify that only one message is published.
• Now change the vehicle's mode and verify that a new message has been published.
• Arm and takeoff, verify that a new message has been published only when armed flag changes.

Test a failsafe:

• Change the parameter BATT_LOW_VOLT to 11.0
• Set the parameter SIM_BATT_VOLTAGE to 10.6.
• Verify that the status topics reports a failsafe of type 22
• Disable the RC setting SIM_RC_FAIL to 1.
• Verify that the failsafe array now reports 22 and 21

[Ryanf55]

Clean commits before merge.

[tizianofiorenzani]

I applied all the suggestions and tested in SITL/Plane again. Here is when a Low battery and RC failsafe has been triggered: