APR I/O code and AuxDyn code refactoring to prepare for RNTuple
Combined APR I/O code and AuxDyn code refactoring, cleanup and improvements done in order to prepare Athena for the new RNTuple APR Storage Technology - which will be added in a separate MR.
Biggest change here - AuxDyn attribute writing code was moved from APR/RootStorageSvc into the more independent AthenaRoot/RootAuxDynIO package where the AuxDyn reading code was relocated already some years ago.