R1:70845a2d13fc

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

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