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/sftp/sftp.go

@@ -59,13 +59,15 @@ func init() {
 		Description: "SSH/SFTP",
 		NewFs:       NewFs,
 		Options: []fs.Option{{
-			Name:     "host",
-			Help:     "SSH host to connect to.\n\nE.g. \"example.com\".",
-			Required: true,
+			Name:      "host",
+			Help:      "SSH host to connect to.\n\nE.g. \"example.com\".",
+			Required:  true,
+			Sensitive: true,
 		}, {
-			Name:    "user",
-			Help:    "SSH username.",
-			Default: currentUser,
+			Name:      "user",
+			Help:      "SSH username.",
+			Default:   currentUser,
+			Sensitive: true,
 		}, {
 			Name:    "port",
 			Help:    "SSH port number.",
@@ -75,8 +77,9 @@ func init() {
 			Help:       "SSH password, leave blank to use ssh-agent.",
 			IsPassword: true,
 		}, {
-			Name: "key_pem",
-			Help: "Raw PEM-encoded private key.\n\nIf specified, will override key_file parameter.",
+			Name:      "key_pem",
+			Help:      "Raw PEM-encoded private key.\n\nIf specified, will override key_file parameter.",
+			Sensitive: true,
 		}, {
 			Name: "key_file",
 			Help: "Path to PEM-encoded private key file.\n\nLeave blank or set key-use-agent to use ssh-agent." + env.ShellExpandHelp,
@@ -87,6 +90,7 @@ func init() {
 Only PEM encrypted key files (old OpenSSH format) are supported. Encrypted keys
 in the new OpenSSH format can't be used.`,
 			IsPassword: true,
+			Sensitive:  true,
 		}, {
 			Name: "pubkey_file",
 			Help: `Optional path to public key file.