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

fs/registry.go

@@ -154,6 +154,7 @@ type Option struct {
 	NoPrefix   bool             // set if the option for this should not use the backend prefix
 	Advanced   bool             // set if this is an advanced config option
 	Exclusive  bool             // set if the answer can only be one of the examples (empty string allowed unless Required or Default is set)
+	Sensitive  bool             // set if this option should be redacted when using rclone config redacted
 }
 
 // BaseOption is an alias for Option used internally