R1:cd84e53c4413

Begin building out RepositoryIdentity indirection layer

Summary: Ref T12164. Start building initial objects for managing `RepositoryIdentity` objects. This won't land until much more of the infrastructure is in place.

Test Plan: Ran `bin/storage upgrade` and observed expected table.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T12164

Differential Revision: https://secure.phabricator.com/D19423