[controller] Implement native Google login (configurable as an alternative to sso-provider)
Implement an OpenID Connect login mechanism that supports (at least) Google login.Cluster configuration should make it possible to
- continue using sso-provider as before (default), or
- use the new OpenID Connect mechanism to sign in with Google.
- Offering the user a backend chooser
- Supporting both sso-provider and OpenID Connect at the same time
- Supporting multiple backends at the same time