add MAVLink FLIGHT_INFORMATION support

[jianyang1]

It would be great if FLIGHT_INFORMATION could be implement in PX4 using MAV_CMD_REQUEST_FLIGHT_INFORMATION in order to calculate flight time from drone messages

right now QGC has flight timer support (mavlink/qgroundcontrol#5100 )- but exiting the software will reset that timer which doesnt make this widget so reliable

[TSC21]

@jianyang1 do you plan to implement this?

[sugnanprabhu]

I'm trying to add support for MAV_CMD_REQUEST_FLIGHT_INFORMATION, is there a better approach to get arming_time and takeoff_time other than adding a new variables to maintain the same.