add preffered_username to idToken

Signed-off-by: Nandor Kracser <bonifaido@gmail.com>

storage/storage.go

@@ -137,10 +137,11 @@ type Client struct {
 
 // Claims represents the ID Token claims supported by the server.
 type Claims struct {
-	UserID        string
-	Username      string
-	Email         string
-	EmailVerified bool
+	UserID            string
+	Username          string
+	PreferredUsername string
+	Email             string
+	EmailVerified     bool
 
 	Groups []string
 }