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/b2/b2.go

@@ -75,13 +75,15 @@ func init() {
 		Description: "Backblaze B2",
 		NewFs:       NewFs,
 		Options: []fs.Option{{
-			Name:     "account",
-			Help:     "Account ID or Application Key ID.",
-			Required: true,
+			Name:      "account",
+			Help:      "Account ID or Application Key ID.",
+			Required:  true,
+			Sensitive: true,
 		}, {
-			Name:     "key",
-			Help:     "Application Key.",
-			Required: true,
+			Name:      "key",
+			Help:      "Application Key.",
+			Required:  true,
+			Sensitive: true,
 		}, {
 			Name:     "endpoint",
 			Help:     "Endpoint for the service.\n\nLeave blank normally.",