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_test.go

@@ -5331,7 +5331,11 @@ func TestInvalidCommitSigError(t *testing.T) {
 
 	// Before the signature gets to Bob, we'll mutate it, such that the
 	// signature is now actually invalid.
-	aliceSig[0] ^= 88
+	aliceSigCopy := aliceSig.Copy()
+	aliceSigCopyBytes := aliceSigCopy.RawBytes()
+	aliceSigCopyBytes[0] ^= 88
+	aliceSig, err = lnwire.NewSigFromWireECDSA(aliceSigCopyBytes)
+	require.NoError(t, err)
 
 	// Bob should reject this new state, and return the proper error.
 	err = bobChannel.ReceiveNewCommitment(aliceSig, aliceHtlcSigs)