Login v2 API

Introducing the v2 Login API, backed by the original Admin.Login logic. Builds on proposed pull request #367.
For pre-1.20 (facade-aware versions), I will have the client detect this and degrade to login V0. This is easy enough to do based on the lack of a Facades map in the response.

I think that should be enough to cover the cases you're concerned about, but let me know if there's another angle to this I'm missing.