Ability to resolve principal name from ExternalIdentityRef without IDP roundtrip

Currently the only way to reliably determine the principal name for a given external identity is by calling ExternalIdentity.getPrincipalName(). This also means that there is currently no way to resolve the principal name from a given ExternalIdentityRef, without calling ExternalIdentityProvider.getIdentity(ExternalIdentityRef).

In the default sync mode a given identity-ref will always be resolved to the associated identity once a given identity is up for (re)sync and thus the identity resolution is part of the synchronization. On the other hand the partial sync as provided by the DynamicSyncContext doesn't require the resolution of group identities but only needs to be able to obtain the principal name, which is needed to proper populate the subject upon repository login (and for permission setup for those group principals). In this setup it would be preferrable if the principal name could be resolved from the ExternalIdentityRef without the intermediate identity resolution.

This aim of this issue is to discuss the different options on how to achieve this improvement in a generic way that doesn't make any assumptions regarding the relationship between ExternalIdentity.getId, ExternalIdentity.getPrincipalName and ExternalIdentityRef.getId.

See also OAK-4930 and OAK-5200 for additional information.