APR I/O code and AuxDyn code refactoring to prepare for RNTuple

Merged Marcin Jerzy Nowak requested to merge mnowak/athena:APR.AuxDynIO.refactor into master

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.

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