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.

lnwallet/channel.go

@@ -320,7 +320,7 @@ type PaymentDescriptor struct {
 	// local node. This signature is generated by the remote node and
 	// stored by the local node in the case that local node needs to
 	// broadcast their commitment transaction.
-	sig *ecdsa.Signature
+	sig input.Signature
 
 	// addCommitHeight[Remote|Local] encodes the height of the commitment
 	// which included this HTLC on either the remote or local commitment
@@ -4331,7 +4331,7 @@ func genHtlcSigValidationJobs(localCommitmentView *commitment,
 		var (
 			htlcIndex uint64
 			sigHash   func() ([]byte, error)
-			sig       *ecdsa.Signature
+			sig       input.Signature
 			err       error
 		)