Support gliding setpoints for autonomous soaring (#14643)

* Enable offboard gliding position setpoints

Adds a capability to make the vehicle glide when given position setpoints.

Gliding setpoints are enabled by setting POSITION_TARGET_TYPEMASK_Z_IGNORE, POSITION_TARGET_TYPEMASK_VZ_IGNORE,  POSITION_TARGET_TYPEMASK_AZ_IGNORE on the typemask of position setpoints.
Include zero cruising throttle
Invalid setpoints with nan

* Handle nans for curise_throttle

src/modules/navigator/navigator_main.cpp

@@ -907,7 +907,7 @@ Navigator::get_cruising_throttle()
 		return _mission_throttle;
 
 	} else {
-		return -1.0f;
+		return NAN;
 	}
 }