Make external account identifier APIs return multiple identifiers

Summary:
Depends on D21012. Ref T13493. Currently, auth adapters return a single identifier for each external account.

Allow them to return more than one identifier, to better handle cases where an API changes from providing a lower-quality identifier to a higher-quality identifier.

On its own, this change doesn't change any user-facing behavior.

Test Plan: Linked and unlinked external accounts.

Maniphest Tasks: T13493

Differential Revision: https://secure.phabricator.com/D21013