lnwallet: only set funding TX witness if we publish

During the final part of the channel funding negotiation we only need to
assemble the full funding TX with the witness if we are going to publish
the transaction ourselves. If the final funding TX is published
externally we don't need this information. This will make it possible to
skip the verify process for fully externally funded PSBT channels.

lnwallet/chanfunding/psbt_assembler.go

@@ -139,6 +139,11 @@ type PsbtIntent struct {
 	// NOTE: This channel must always be buffered.
 	PsbtReady chan error
 
+	// shouldPublish specifies if the intent assumes its assembler should
+	// publish the transaction once the channel funding has completed. If
+	// this is set to false then the finalize step can be skipped.
+	shouldPublish bool
+
 	// signalPsbtReady is a Once guard to make sure the PsbtReady channel is
 	// only closed exactly once.
 	signalPsbtReady sync.Once
@@ -449,6 +454,12 @@ func (i *PsbtIntent) Outputs() []*wire.TxOut {
 	}
 }
 
+// ShouldPublishFundingTX returns true if the intent assumes that its assembler
+// should publish the funding TX once the funding negotiation is complete.
+func (i *PsbtIntent) ShouldPublishFundingTX() bool {
+	return i.shouldPublish
+}
+
 // PsbtAssembler is a type of chanfunding.Assembler wherein the funding
 // transaction is constructed outside of lnd by using partially signed bitcoin
 // transactions (PSBT).
@@ -500,10 +511,11 @@ func (p *PsbtAssembler) ProvisionChannel(req *Request) (Intent, error) {
 		ShimIntent: ShimIntent{
 			localFundingAmt: p.fundingAmt,
 		},
-		State:     PsbtShimRegistered,
-		BasePsbt:  p.basePsbt,
-		PsbtReady: make(chan error, 1),
-		netParams: p.netParams,
+		State:         PsbtShimRegistered,
+		BasePsbt:      p.basePsbt,
+		PsbtReady:     make(chan error, 1),
+		shouldPublish: p.shouldPublish,
+		netParams:     p.netParams,
 	}
 
 	// A simple sanity check to ensure the provisioned request matches the