context: define a constant for transaction context type

pkg/smartcontract/context/context.go

@@ -21,6 +21,12 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/wallet"
 )
 
+// TransactionType is the ParameterContext Type used for transactions.
+const TransactionType = "Neo.Network.P2P.Payloads.Transaction"
+
+// compatTransactionType is the old, 2.x type used for transactions.
+const compatTransactionType = "Neo.Core.ContractTransaction"
+
 // ParameterContext represents smartcontract parameter's context.
 type ParameterContext struct {
 	// Type is a type of a verifiable item.
@@ -213,7 +219,7 @@ func (c *ParameterContext) UnmarshalJSON(data []byte) error {
 
 	var verif crypto.VerifiableDecodable
 	switch pc.Type {
-	case "Neo.Core.ContractTransaction", "Neo.Network.P2P.Payloads.Transaction":
+	case compatTransactionType, TransactionType:
 		tx := new(transaction.Transaction)
 		verif = tx
 	default:

pkg/smartcontract/context/context_test.go

@@ -102,7 +102,7 @@ func TestParameterContext_AddSignatureMultisig(t *testing.T) {
 		},
 	}
 	tx := getContractTx(ctr.ScriptHash())
-	c := NewParameterContext("Neo.Network.P2P.Payloads.Transaction", netmode.UnitTestNet, tx)
+	c := NewParameterContext(TransactionType, netmode.UnitTestNet, tx)
 	priv, err := keys.NewPrivateKey()
 	require.NoError(t, err)
 	sig := priv.SignHashable(uint32(c.Network), tx)