Plane: airbrakes using dual rudder mixing

See example F-15 Tomcat where the rear dual rudders could be used for airbrakes
https://www.youtube.com/watch?v=7rvGGwGEUXU&feature=youtu.be&t=3054

Here's the FB post
https://www.facebook.com/groups/ArduPilot.org/permalink/1812223075467544/

Start by reviewing the linked F-15 example video and Facebook post to clarify the intended airbrake behavior. Then locate the ArduPlane flight-control and rudder-mixing entry points, since no files or tests are named here. Done requires an agreed mixing design and verification that dual rudders provide the intended airbrake effect without disrupting normal control.

Written by the indexing model from the issue text.