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