[wip] gz optical flow

[dakejahl]

continuation of #23456

Problem
Quads with optical flow can't seem to reliably takeoff (get stuck on the ground) and if they do manage to takeoff end up at the wrong altitude. It seems that the Local Position Z estimate is being miscalculated. The drone will be sitting on the ground with props spinning, and the local position Z continues to increase.

Steps to Reproduce

make px4_sitl_default gz_x500_flow_aruco

Issue a takeoff command

pxh> commander takeoff
pxh> INFO  [commander] Armed by internal command	
INFO  [tone_alarm] arming warning
INFO  [navigator] Using default takeoff altitude: 2.5 m	
INFO  [commander] Takeoff detected

Watch the vehicle stay on the ground

Testing
Here's a log from sim. In this test I followed the above steps. The drone did eventually lift off the ground briefly to ~0.3m, before descending again. I did not intervene at all during this test until issuing a land command at the end when the drone was already back on the ground.
https://review.px4.io/plot_app?log=40b7d570-33af-4225-ac11-927feea2aa94

Expectation
The vehicle should takeoff to 2.5m