auth: Help consumers manage signing into tenants better

[alexweininger]

Consumers of the auth package are running into the same issues that made us develop the tenants view. Users sign in but we can't list subscriptions because we need users to sign in to a specific tenant. For now, consuming extensions need to handle this on their own, but ideally we could handle this in a central place.

[alexweininger]

Related to #1815