Add R4 version of MDT hough transform

Add a first version of the Muon Hough transform for the WIP Phase-2 muon reconstruction. Use ACTS as back-end, while detector-specific work is handled in athena.

Also add validation package and required event data classes.

Documentation has been presented in the ITK Acts integration meeting.

Large MR since required ACTS version in Externals only recently arrived. Further development will happen in smaller MR.