Draft: Tau GNN data loading with FTag SaltModel infrastructure

Hi All,

This MR updates the TauGNN to use the SaltModel EDM loading infrastructure.

Detail of the motivation and discussions can be found in https://indico.cern.ch/event/1595490/

Open items:

• CPU overhead estimation with this MR.
• Larger scale validation beyond CI.

Marking as draft as this is still in development.