The StartLoginInteractive command is for delegating the sign-in flow
to a browser. The Android Gooogle Sign-In SDK inverts the flow by
giving the client ID tokens.
Add a new backend command for accepting such tokens by exposing the existing
controlclient.Client.Login support for OAuth2 tokens. Introduce a custom
TokenType to distinguish ID tokens from other OAuth2 tokens.
Signed-off-by: Elias Naur <mail@eliasnaur.com>