multi: Add --tor.encryptkey flag functionality to encrypt the Tor private key on disk

It's possible that a user might not want the Tor private key to sit on the disk in plaintext (it is a private key after all). So this commit adds a new flag to encrypt the Tor private key on disk using the wallet's seed. When the --tor.encryptkey flag is used, LND will still write the Tor key to the same file, however it will now be encrypted intead of plaintext. This essentially uses the same method to encrypt the Tor private key as is used to encrypt the Static Channel Backup file.

watchtower/config.go

@@ -98,6 +98,12 @@ type Config struct {
 	// for a watchtower hidden service should be stored.
 	WatchtowerKeyPath string
 
+	// EncryptKey will encrypt the Tor private key on disk.
+	EncryptKey bool
+
+	// KeyRing is the KeyRing to use when encrypting the Tor private key.
+	KeyRing keychain.KeyRing
+
 	// Type specifies the hidden service type (V2 or V3) that the watchtower
 	// will create.
 	Type tor.OnionType