xAODRootAccess: fix type information for ConstDataVector holder

Frank Winklmeierrequested to merge
fwinkl/athena:tcdvholder into master

In case of a ConstDataVector use the type information of the underlying DataVector so it can be retrieved by clients. See the discussion on !55866 (comment 6164480).

Also add public accessor for the holder to TStore.

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