Summary:
Depends on D21010. Ref T13493. External accounts may have multiple different unique identifiers, most often when v1 of the API makes a questionable choice (and provies a mutable, non-unique, or PII identifier) and v2 of the API uses an immutable, unique, random identifier.
Allow Phabricator to store multiple identifiers per external account.
Test Plan: Storage only, see followup changes.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13493
Differential Revision: https://secure.phabricator.com/D21011
