multi: introduce and use new TapTweak and SignMethod fields

In this commit, we add a new field `TapTweak` to be used for key path
spends. Before this commit, we'd overload the existing `WitnessScript`
field to pass this information to the signing context. This was
confusing as for tapscript spends, this was the leaf script, which
mirrors the other script based spending types.

With this new filed, users need to set this to the script root for
keypath spends where the output key commits to a real merkle root, and
nothing when bip 86 spending is being used.

To make the signing even more explicit, we also add a new field called
sign_method with an enum type that differentiates between the different
segwit v0 and v1 signing methods.

Fixes https://github.com/lightningnetwork/lnd/issues/6446.

lnwallet/btcwallet/signer.go

@@ -372,21 +372,23 @@ func (b *BtcWallet) SignOutputRaw(tx *wire.MsgTx,
 		// slightly different, so we need to account for that to get the
 		// raw signature.
 		var rawSig []byte
-		if signDesc.TaprootKeySpend {
+		switch signDesc.SignMethod {
+		case input.TaprootKeySpendBIP0086SignMethod,
+			input.TaprootKeySpendSignMethod:
+
 			// This function tweaks the private key using the tap
-			// root key supplied as the tweak. So we pass in the
-			// original private key to avoid it being double
-			// tweaked!
+			// root key supplied as the tweak.
 			rawSig, err = txscript.RawTxInTaprootSignature(
 				tx, sigHashes, signDesc.InputIndex,
 				signDesc.Output.Value, signDesc.Output.PkScript,
-				signDesc.WitnessScript, signDesc.HashType,
+				signDesc.TapTweak, signDesc.HashType,
 				privKey,
 			)
 			if err != nil {
 				return nil, err
 			}
-		} else {
+
+		case input.TaprootScriptSpendSignMethod:
 			leaf := txscript.TapLeaf{
 				LeafVersion: txscript.BaseLeafVersion,
 				Script:      witnessScript,