multi: add new tapscript root option to GenTaprootFundingScript

This'll allow us to create a funding output that uses musig2, but uses a tapscript tweak rather than a normal BIP 86 tweak.

itest/lnd_funding_test.go

@@ -12,6 +12,7 @@ import (
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/chainreg"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/funding"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/labels"
@@ -1192,6 +1193,7 @@ func deriveFundingShim(ht *lntest.HarnessTest, carol, dave *node.HarnessNode,
 
 		_, fundingOutput, err = input.GenTaprootFundingScript(
 			carolKey, daveKey, int64(chanSize),
+			fn.None[chainhash.Hash](),
 		)
 		require.NoError(ht, err)