walletrpc+lnwallet: possibility to define an address type for changes in FundPsbt

By default, P2TR addresses are used for changes. However, some users
might encounter some problems with this change. We add the possibility
to define a custom address type in FundPsbt for default/imported accounts
(only P2TR for now). If no address type is specified for these accounts,
we will use P2WKH by default.

lnrpc/walletrpc/walletkit_server.go

@@ -1023,7 +1023,11 @@ func (w *WalletKit) LabelTransaction(ctx context.Context,
 // externally and no additional inputs are added. If the specified inputs aren't
 // enough to fund the outputs with the given fee rate, an error is returned.
 // After either selecting or verifying the inputs, all input UTXOs are locked
-// with an internal app ID.
+// with an internal app ID. A custom address type for change can be specified
+// for default accounts and single imported public keys (only P2TR for now).
+// Otherwise, P2WPKH will be used by default. No custom address type should be
+// provided for custom accounts as we will always generate the change address
+// using the coin selection key scope.
 //
 // NOTE: If this method returns without an error, it is the caller's
 // responsibility to either spend the locked UTXOs (by finalizing and then
@@ -1178,6 +1182,7 @@ func (w *WalletKit) FundPsbt(_ context.Context,
 		// generating a new change address.
 		changeIndex, err = w.cfg.Wallet.FundPsbt(
 			packet, minConfs, feeSatPerKW, account,
+			keyScopeFromChangeAddressType(req.ChangeType),
 		)
 		if err != nil {
 			return fmt.Errorf("wallet couldn't fund PSBT: %v", err)
@@ -1243,6 +1248,21 @@ func marshallLeases(locks []*base.ListLeasedOutputResult) []*UtxoLease {
 	return rpcLocks
 }
 
+// keyScopeFromChangeAddressType maps a ChangeAddressType from protobuf to a
+// KeyScope. If the type is ChangeAddressType_CHANGE_ADDRESS_TYPE_UNSPECIFIED,
+// it returns nil.
+func keyScopeFromChangeAddressType(
+	changeAddressType ChangeAddressType) *waddrmgr.KeyScope {
+
+	switch changeAddressType {
+	case ChangeAddressType_CHANGE_ADDRESS_TYPE_P2TR:
+		return &waddrmgr.KeyScopeBIP0086
+
+	default:
+		return nil
+	}
+}
+
 // SignPsbt expects a partial transaction with all inputs and outputs fully
 // declared and tries to sign all unsigned inputs that have all required fields
 // (UTXO information, BIP32 derivation information, witness or sig scripts)