APM_Control: add AP_FW_Controller as common base class to roll and pitch controlers

[IamPete1]

This moves to a shared base class between the plane roll and pitch controllers, there is lots of shared code which was duplicated. This helps with future tidyups as they now only need to happen in one place.

[IamPete1]

This is a example of something it would simplify. #27694

[peterbarker]

I tried this on for size and got bounced: #22023

"Needs a lot of testing" was not the only comment made.

From the meeting notes:

UTC0016 - [APM_Control: factor out an AP_PitchRollController base class by peterbarker · Pull Request #22023 · ArduPilot/ardupilot · GitHub](https://github.com/ArduPilot/ardupilot/pull/22023/files)

    Parameter conversion
    Check scale factor
    Pitch tracking in TECS

... and it was also stated that increasing complexity in this area was not wanted and burning those extra bytes was fine.

[timtuxworth]

This is awesome. It's a very nice structural change that should make some other things much easier in future.