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.

lnwallet/channel.go

@@ -1013,6 +1013,7 @@ func (lc *LightningChannel) createSignDesc() error {
 	if chanState.ChanType.IsTaproot() {
 		fundingPkScript, _, err = input.GenTaprootFundingScript(
 			localKey, remoteKey, int64(lc.channelState.Capacity),
+			fn.None[chainhash.Hash](),
 		)
 		if err != nil {
 			return err