Permit multiple login options to the same Dataverse account

[pdurbin]

The original design doc at https://github.com/IQSS/dataverse/blob/4.0/doc/Architecture/auth.md was for "allow multiple logins to map to the same internal system user" but at the "auth branch merge party" in late September 2014 we decided to do away with this concept in fe8e41a. We'd like to at least explore the possibility of reintroducing the idea of permitting multiple login options to the same Dataverse account. The idea is that you could log in with HarvardKey (Shibboleth) or ORCID or a local account and still be "you" in Dataverse.

[pdurbin]

I think we want this, but when? Closing until we decide to prioritize it.