rclone config redacted: implement support mechanism for showing redacted config

This introduces a new fs.Option flag, Sensitive and uses this along
with IsPassword to redact the info in the config file for support
purposes.

It adds this flag into backends where appropriate. It was necessary to
add oauthutil.SharedOptions to some backends as they were missing
them.

Fixes #5209

backend/dropbox/dropbox.go

@@ -182,8 +182,9 @@ client_secret) to use this option as currently rclone's default set of
 permissions doesn't include "members.read". This can be added once
 v1.55 or later is in use everywhere.
 `,
-			Default:  "",
-			Advanced: true,
+			Default:   "",
+			Advanced:  true,
+			Sensitive: true,
 		}, {
 			Name: "shared_files",
 			Help: `Instructs rclone to work on individual shared files.