Merge branch 'set_federated_username' into 'master'

Define userName variable distinct from eppn in federated mapper

This change is motivated by the need to set userName to a value other than default value of eppn for federated preauthenticated users.  The intended use case is UW Digital ID, where we plan to use SPVI as the username for the purposes of the RequestOnBehalfOfFilter.  

@paul.erickson @bkeen @npblair 

See merge request !16