lnwallet/chanfunding: add Inputs/Outputs to assemblers

This will be used to try to estimate how much the funding transaction
will decrease our wallet balance.

lnwallet/chanfunding/canned_assembler.go

@@ -98,6 +98,30 @@ func (s *ShimIntent) ThawHeight() uint32 {
 	return s.thawHeight
 }
 
+// Inputs returns all inputs to the final funding transaction that we
+// know about. For the ShimIntent this will always be none, since it is funded
+// externally.
+func (s *ShimIntent) Inputs() []wire.OutPoint {
+	return nil
+}
+
+// Outputs returns all outputs of the final funding transaction that we
+// know about. Since this is an externally funded channel, the channel output
+// is the only known one.
+func (s *ShimIntent) Outputs() []*wire.TxOut {
+	_, txOut, err := s.FundingOutput()
+	if err != nil {
+		log.Warnf("Unable to find funding output for shim intent: %v",
+			err)
+
+		// Failed finding funding output, return empty list of known
+		// outputs.
+		return nil
+	}
+
+	return []*wire.TxOut{txOut}
+}
+
 // FundingKeys couples our multi-sig key along with the remote party's key.
 type FundingKeys struct {
 	// LocalKey is our multi-sig key.