multi: allow skipping the PSBT finalize step

The FundingPsbtFinalize step is a safety measure that assures the final
signed funding transaction has the same TXID as was registered during
the funding flow and was used for the commitment transactions.
This step is cumbersome to use if the whole funding process is completed
external to lnd. We allow the finalize step to be skipped for such
cases. The API user/script will need to make sure things are verified
(and possibly cleaned up) properly.

funding/batch.go

@@ -126,7 +126,7 @@ type Wallet interface {
 	// PsbtFundingVerify looks up a previously registered funding intent by
 	// its pending channel ID and tries to advance the state machine by
 	// verifying the passed PSBT.
-	PsbtFundingVerify([32]byte, *psbt.Packet) error
+	PsbtFundingVerify([32]byte, *psbt.Packet, bool) error
 
 	// PsbtFundingFinalize looks up a previously registered funding intent
 	// by its pending channel ID and tries to advance the state machine by
@@ -355,7 +355,7 @@ func (b *Batcher) BatchFund(ctx context.Context,
 	// each of the channels.
 	for _, channel := range b.channels {
 		err = b.cfg.Wallet.PsbtFundingVerify(
-			channel.pendingChanID, unsignedPacket,
+			channel.pendingChanID, unsignedPacket, false,
 		)
 		if err != nil {
 			return nil, fmt.Errorf("error verifying PSBT: %v", err)