oauthutil: adds SharedOptions for OAuth backends

1. adds SharedOptions data structure to oauthutil
2. adds config.ConfigToken option to oauthutil.SharedOptions
3. updates the backends that have oauth functionality

Fixes #2849

backend/googlecloudstorage/googlecloudstorage.go

@@ -88,13 +88,7 @@ func init() {
 				log.Fatalf("Failed to configure token: %v", err)
 			}
 		},
-		Options: []fs.Option{{
-			Name: config.ConfigClientID,
-			Help: "Google Application Client Id\nLeave blank normally.",
-		}, {
-			Name: config.ConfigClientSecret,
-			Help: "Google Application Client Secret\nLeave blank normally.",
-		}, {
+		Options: append(oauthutil.SharedOptions, []fs.Option{{
 			Name: "project_number",
 			Help: "Project number.\nOptional - needed only for list/create/delete buckets - see your developer console.",
 		}, {
@@ -261,7 +255,7 @@ Docs: https://cloud.google.com/storage/docs/bucket-policy-only
 			Default: (encoder.Base |
 				encoder.EncodeCrLf |
 				encoder.EncodeInvalidUtf8),
-		}},
+		}}...),
 	})
 }