Cluster EDM converters as global functions
This introduces EDM converters as global functions. This MR only addresses xAOD::Clusters
Follow-up MR will use these converters in the ACTS tracking code, removing the need (for the CKF at least) to retrieve the InDet cluster link from the xAOD cluster link. This link is only set during the EDM cluster convertion, which is not supposed to be scheduled in production mode. As a result, the ACTS CKF currently required InDet clustering + EDM convertion. The ACTS clustering algorithm cannot be scheduled.
Once these functions will be deployed, the CKF will be able to run with the ACTS clustering algorithm, removing the need for the EDM convertion.