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/cache/cache.go

@@ -76,17 +76,19 @@ func init() {
 			Name: "plex_url",
 			Help: "The URL of the Plex server.",
 		}, {
-			Name: "plex_username",
-			Help: "The username of the Plex user.",
+			Name:      "plex_username",
+			Help:      "The username of the Plex user.",
+			Sensitive: true,
 		}, {
 			Name:       "plex_password",
 			Help:       "The password of the Plex user.",
 			IsPassword: true,
 		}, {
-			Name:     "plex_token",
-			Help:     "The plex token for authentication - auto set normally.",
-			Hide:     fs.OptionHideBoth,
-			Advanced: true,
+			Name:      "plex_token",
+			Help:      "The plex token for authentication - auto set normally.",
+			Hide:      fs.OptionHideBoth,
+			Advanced:  true,
+			Sensitive: true,
 		}, {
 			Name:     "plex_insecure",
 			Help:     "Skip all certificate verification when connecting to the Plex server.",