Add an "ExternalAccountIdentifier" table

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