need to vet parameter set() calls

[tridge]

we have quite a lot of code using .set() on parameters to temporarily disable features. Examples in mode_turtle and motor_test in copter with failsafes. if the user fetches parameters while these operations are happening they will end up with a param set without RC failsafe enabled, which if loaded later could be dangerous

[IamPete1]

https://github.com/ArduPilot/ardupilot/issues/20788

[rmackay9]

As discussed on the call perhaps we could create an new AP_Param type that creates a shadow variable that is used from within the C++ code when .get() or .set() are called but not used for the param download.