Heli autorotation restructure mode and use position controller

[MattKear]

This PR is the next step in bringing on the autorotation flight mode. It builds on the work done in #27786 and is dependant upon it. Hence, the diff looks very large at the moment.

The following changes have been made:

• A much needed re-structure to move more of the code out of the mode and down into the autorotation library
• Add pre-arm checks specific to the autorotation mode
• Implement the position controller for XY control

This has been tested IRL and is working.

Nothing in this PR should effect real vehicles as this predominantly changes the SITL only autorotation mode.