repos: add rmackay9-opticalflow

[rmackay9]

This adds the rmackay9-opticalflow repository to the list of publicly available blueos extensions

The code for the extension can be seen here although I'd like to move it (and the one for precision landing) to the https://github.com/BlueOS-community repo in the near future to make it easier for other devs to contribute changes.

This is what the extension looks like to the user.

I've also flight tested it on a real vehicle up to 20m and it performs well. I suspect it will work at much higher altitudes as well but that was the limit of my rangefinder. I'll need a longer rangefinder and/or change AP a bit to allower optical flow without a rangefinder (e.g. switch to barometer and just estimate the alt-above-terrain).

Special thanks to Sanket Sharma for helping me with the optical flow algorithm

[rmackay9]

@patrickelectric, thanks very much!