lnwire: update Sig to support both ECDSA and schnorr sigs

In this commit, we update the Sig type to support ECDSA and schnorr
signatures. We need to do this as the HTLC signatures will become
schnorr sigs for taproot channels. The current spec draft opts to
overload this field since both the sigs are actually 64 bytes in length.
The only consideration with this move is that callers need to "coerce" a
sig to the proper type if they need schnorr signatures.

watchtower/wtclient/backup_task.go

@@ -369,24 +369,26 @@ func (t *backupTask) craftSessionPayload(
 
 		// Re-encode the DER signature into a fixed-size 64 byte
 		// signature.
-		signature, err := lnwire.NewSigFromRawSignature(rawSignature)
+		signature, err := lnwire.NewSigFromECDSARawSignature(
+			rawSignature,
+		)
 		if err != nil {
 			return hint, nil, err
 		}
 
 		// Finally, copy the serialized signature into the justice kit,
 		// using the input's witness type to select the appropriate
-		// field.
+		// field
 		switch inp.WitnessType() {
 		case input.CommitmentRevoke:
-			copy(justiceKit.CommitToLocalSig[:], signature[:])
+			justiceKit.CommitToLocalSig = signature
 
 		case input.CommitSpendNoDelayTweakless:
 			fallthrough
 		case input.CommitmentNoDelay:
 			fallthrough
 		case input.CommitmentToRemoteConfirmed:
-			copy(justiceKit.CommitToRemoteSig[:], signature[:])
+			justiceKit.CommitToRemoteSig = signature
 		default:
 			return hint, nil, fmt.Errorf("invalid witness type: %v",
 				inp.WitnessType())