msg : add ping message

msg/CMakeLists.txt

@@ -77,6 +77,7 @@ set(msg_files
 	offboard_control_mode.msg
 	optical_flow.msg
 	parameter_update.msg
+	ping.msg
 	position_setpoint.msg
 	position_setpoint_triplet.msg
 	power_button_state.msg

msg/ping.msg

+uint64 ping_time			# Timestamp of the ping packet
+uint32 ping_sequence		# Sequence number of the ping packet
+uint32 dropped_packets		# Number of dropped ping packets
+float32 rtt_ms				# Round trip time (in ms)
+uint8 system_id				# System ID of the remote system
+uint8 component_id			# Component ID of the remote system