modifyRegistration API needs to support AlternativePersistentIdentifiers as well

[landreev]

In the HDV-specific use case, attempting to re-register one of our legacy handles, for example:

curl ... -X POST "http://localhost:8080/api/datasets/:persistentId/modifyRegistration?persistentId=hdl:1902.1/00021"

will "work", but only in a sense that it will modify the registration of the current main persistent id of the dataset above, which is doi:10.7910/DVN/4MRH2N. This is because the API uses findDatasetOrDie() to look up the dataset ... etc.

It would be useful to be able re-register or update registrations for alt. pids as well. As of now, whenever we need to modify our legacy handles, it has to be done outside Dataverse and requires manual work. This will be a little less trivial than just replacing findDatasetOrDie() with 2 separate lookups, or creating another dedicated API (I'm pretty sure the registration code defaults to the primary pid of the dataset further down as well). But should be very doable.