scale yaw headroom with throttle

We've had a request to scale Q_M_YAW_HEADROOM with throttle, so when climbing use a lower headroom value to get a better climb rate, particularly in gusty conditions, but when descending use the full yaw headroom
This could be done with a lua script, but may be worthwhile as a builtin feature?
@lthall ?

Start by locating Q_M_YAW_HEADROOM and the throttle-related flight-control entry points; the issue does not name specific files or tests. Compare the proposed Lua-script approach with a built-in feature, and define done as scaling yaw headroom lower while climbing and using full headroom while descending, with behavior validated in gusty conditions.

Written by the indexing model from the issue text.