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.

zpay32/encode.go

@@ -91,8 +91,10 @@ func (invoice *Invoice) Encode(signer MessageSigner) (string, error) {
 	// From the header byte we can extract the recovery ID, and the last 64
 	// bytes encode the signature.
 	recoveryID := sign[0] - 27 - 4
-	var sig lnwire.Sig
-	copy(sig[:], sign[1:])
+	sig, err := lnwire.NewSigFromWireECDSA(sign[1:])
+	if err != nil {
+		return "", err
+	}
 
 	// If the pubkey field was explicitly set, it must be set to the pubkey
 	// used to create the signature.
@@ -112,7 +114,10 @@ func (invoice *Invoice) Encode(signer MessageSigner) (string, error) {
 	}
 
 	// Convert the signature to base32 before writing it to the buffer.
-	signBase32, err := bech32.ConvertBits(append(sig[:], recoveryID), 8, 5, true)
+	signBase32, err := bech32.ConvertBits(
+		append(sig.RawBytes(), recoveryID),
+		8, 5, true,
+	)
 	if err != nil {
 		return "", err
 	}