multi: add tag option to SignMessageSchnorr

keychain/derivation.go

@@ -239,8 +239,8 @@ type MessageSignerRing interface {
 	// hashing it first, with the private key described in the key locator
 	// and the optional Taproot tweak applied to the private key.
 	SignMessageSchnorr(keyLoc KeyLocator, msg []byte,
-		doubleHash bool, taprootTweak []byte) (*schnorr.Signature,
-		error)
+		doubleHash bool, taprootTweak []byte,
+		tag []byte) (*schnorr.Signature, error)
 }
 
 // SingleKeyMessageSigner is an abstraction interface that hides the