MuonIdHelpers - Cache the sMDT identifiers during the initialization

Hi,

I noticed that a decent fraction of the time in MdtToRdoPrepData is spent in issMdt from the IdHelperSvc, in particular in the validity check of the Rpc ids. The list of sMDT chambers only changes between run 2 and run 3 or run 3 and run 4. So let's cache the Identifiers of the ElementIDs and navigate back to these to find the sMDT stations.