Explain what you would like to see improved

Users should be allowed to customize the underlying object class of DAOS objects,
depending on their particular scenario. Unless the user specifies otherwise, we will
probably default to OC_SX.

Optional: share how it could be improved

These kind of backend-specific parameters should be exposed via a subclass of
RNTupleWriteOptions, e.g. RNTupleDaosWriteOptions.