AthContainers: Additions for compatibility with main branch.

In main, the accessor classes were moved out of AuxElement and into the SG namespace, being declared in separate headers.

In order to minimize divergence in client code between 24 and main, add some wrappers to allow using the syntax from main in 24. Add typedefs to bring the accessor classes into the SG namespace, and add forwarding headers.

Copied from the r24 branch. See ATLINFR-5539.

mentioned in commit 1573d0d7

merged