input: introduce Signature iface

This commit introduces the Signature interface which will be used by our
witness construction methods instead of passing in raw byte slices. This
will be used later to inject various kinds of mock signatures, e.g.
73-byte signatures for simulating worst-case witness weight.

lnwallet/channel.go

@@ -5459,11 +5459,16 @@ func newOutgoingHtlcResolution(signer input.Signer,
 		InputIndex: 0,
 	}
 
+	htlcSig, err := btcec.ParseDERSignature(htlc.Signature, btcec.S256())
+	if err != nil {
+		return nil, err
+	}
+
 	// With the sign desc created, we can now construct the full witness
 	// for the timeout transaction, and populate it as well.
 	sigHashType := HtlcSigHashType(chanType)
 	timeoutWitness, err := input.SenderHtlcSpendTimeout(
-		htlc.Signature, sigHashType, signer, &timeoutSignDesc, timeoutTx,
+		htlcSig, sigHashType, signer, &timeoutSignDesc, timeoutTx,
 	)
 	if err != nil {
 		return nil, err
@@ -5585,14 +5590,18 @@ func newIncomingHtlcResolution(signer input.Signer,
 		InputIndex: 0,
 	}
 
+	htlcSig, err := btcec.ParseDERSignature(htlc.Signature, btcec.S256())
+	if err != nil {
+		return nil, err
+	}
+
 	// Next, we'll construct the full witness needed to satisfy the input of
 	// the success transaction. Don't specify the preimage yet. The preimage
 	// will be supplied by the contract resolver, either directly or when it
 	// becomes known.
 	sigHashType := HtlcSigHashType(chanType)
 	successWitness, err := input.ReceiverHtlcSpendRedeem(
-		htlc.Signature, sigHashType, nil, signer, &successSignDesc,
-		successTx,
+		htlcSig, sigHashType, nil, signer, &successSignDesc, successTx,
 	)
 	if err != nil {
 		return nil, err