Scoped Auth via non-unique emails on auth.users

[jonkurtis]

It would be great to be able to scope auth by a tenant_id. Currently there is a partial unique index on auth.users email column. I would like to be able to have a single user use the same email to login via supabase auth on different tenants with separate email/password for each. This would enable building platforms that have tenant scoped auth for users. For instance if you were building a newsletter platform the newsletter subscribers could be scoped to a publication so they could use the same email to authenticate for multiple publications.