modifyRegistration API needs to support AlternativePersistentIdentifiers as well

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.

Start at the modifyRegistration API and its findDatasetOrDie() lookup, then trace the registration code to see where it defaults to the dataset's primary persistent identifier. Confirm that an alternative persistent identifier selects the intended dataset and registration while preserving primary-identifier behavior; the issue is done when legacy handles can be re-registered through this API without manual work.