AP_DDS: use generated types for service serialisation

[srmainwaring]

Use the generated types from ardupilot_msgs to de-serialise and serialise the requests and responses for the arm motors and mode switch services. This allows the generated serialisation code to handle byte alignment etc.

Testing

Run the iris runway world:

$ ros2 launch ardupilot_gz_bringup iris_runway.launch.py rviz:=true use_gz_tf:=true

Run MAVProxy:

$ mavproxy.py --master udp:127.0.0.1:14550  --console --moddebug 3

Switch mode to GUIDED (=4)

$ ros2 service call /ap/mode_switch ardupilot_msgs/srv/ModeSwitch "{mode: 4}"   
requester: making request: ardupilot_msgs.srv.ModeSwitch_Request(mode=4)

response:
ardupilot_msgs.srv.ModeSwitch_Response(status=True, curr_mode=4)

Arm motors:

$ ros2 service call /ap/arm_motors ardupilot_msgs/srv/ArmMotors "{arm: true}"
requester: making request: ardupilot_msgs.srv.ArmMotors_Request(arm=True)

response:
ardupilot_msgs.srv.ArmMotors_Response(result=True)