ipn: add Login backend command for sign-in with token

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>

ipn/handle.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/tailscale/wireguard-go/wgcfg"
+	"golang.org/x/oauth2"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/types/logger"
 )
@@ -154,6 +155,10 @@ func (h *Handle) StartLoginInteractive() {
 	h.b.StartLoginInteractive()
 }
 
+func (h *Handle) Login(token *oauth2.Token) {
+	h.b.Login(token)
+}
+
 func (h *Handle) Logout() {
 	h.b.Logout()
 }