Registers a signed token with the user's Media & Purchases account.
- Parameter token: The signed token representing user entitlement.
- Throws: ``RegistrationError/notEligible`` if app is not a registered partner,
``RegistrationError/rateLimited`` if the server is rate limiting requests
(check `userInfo[RegistrationErrorRetryAfterKey]` for the retry delay),
or ``RegistrationError/failed`` for other failures.
Registers an account identifier with the user's Media & Purchases account.
- Parameter identifier: The UUID string representing user entitlement.
- Throws: ``RegistrationError/notEligible`` if app is not a registered partner,
``RegistrationError/rateLimited`` if the server is rate limiting requests
(check `userInfo[RegistrationErrorRetryAfterKey]` for the retry delay),
or ``RegistrationError/failed`` for other failures.