multi: update sweeping to allow different sighashes and claim scripts

Taproot spends require a different sighash, so we update our HtlcScript
interface to provide the appropriate sighash when sweeping. We also add
distinct Timeout/Success Script functions to allow for tapleaf spends
which have different locking scripts for different paths. Note that the
timeout and success paths will be the same for segwit v0 htlcs, because
it has a single branched script containing all spend paths.

In future iterations, this differentiation of claim scripts can also
be used to use musig2 to collaboratively keyspend P2TR htlcs with the
server. This script can be expressed as PriorityScript (because we'll
try to keyspend as a priority, and then fall back to a tap leaf spend).
As we've done here, segwit v0 spends would just return their single
script for PriorityScript, and the claim would be no different from
our other claims.

sweep/sweeper.go

@@ -23,7 +23,7 @@ type Sweeper struct {
 func (s *Sweeper) CreateSweepTx(
 	globalCtx context.Context, height int32, sequence uint32,
 	htlc *swap.Htlc, htlcOutpoint wire.OutPoint,
-	keyBytes [33]byte,
+	keyBytes [33]byte, witnessScript []byte,
 	witnessFunc func(sig []byte) (wire.TxWitness, error),
 	amount, fee btcutil.Amount,
 	destAddr btcutil.Address) (*wire.MsgTx, error) {
@@ -59,20 +59,30 @@ func (s *Sweeper) CreateSweepTx(
 	}
 
 	signDesc := lndclient.SignDescriptor{
-		WitnessScript: htlc.Script(),
+		WitnessScript: witnessScript,
 		Output: &wire.TxOut{
 			Value:    int64(amount),
 			PkScript: htlc.PkScript,
 		},
-		HashType:   txscript.SigHashAll,
+		HashType:   htlc.SigHash(),
 		InputIndex: 0,
 		KeyDesc: keychain.KeyDescriptor{
 			PubKey: key,
 		},
 	}
 
+	// We need our previous outputs for taproot spends, and there's no
+	// harm including them for segwit v0, so we always include our prevOut.
+	prevOut := []*wire.TxOut{
+		{
+			Value:    int64(amount),
+			PkScript: htlc.PkScript,
+		},
+	}
+
 	rawSigs, err := s.Lnd.Signer.SignOutputRaw(
-		globalCtx, sweepTx, []*lndclient.SignDescriptor{&signDesc}, nil,
+		globalCtx, sweepTx, []*lndclient.SignDescriptor{&signDesc},
+		prevOut,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("signing: %v", err)