context: define a constant for transaction context type

cli/paramcontext/context.go

@@ -15,7 +15,7 @@ import (
 // as an input to `multisig sign`. If a wallet.Account is given and can sign,
 // it's signed as well using it.
 func InitAndSave(net netmode.Magic, tx *transaction.Transaction, acc *wallet.Account, filename string) error {
-	scCtx := context.NewParameterContext("Neo.Network.P2P.Payloads.Transaction", net, tx)
+	scCtx := context.NewParameterContext(context.TransactionType, net, tx)
 	if acc != nil && acc.CanSign() {
 		sign := acc.SignHashable(net, tx)
 		if err := scCtx.AddSignature(acc.ScriptHash(), acc.Contract, acc.PublicKey(), sign); err != nil {

pkg/rpcclient/actor/doc_test.go

@@ -108,7 +108,7 @@ func ExampleActor() {
 	tx, _ := policyContract.SetStoragePriceUnsigned(10)
 
 	net := a.GetNetwork()
-	scCtx := sccontext.NewParameterContext("Neo.Network.P2P.Payloads.Transaction", net, tx)
+	scCtx := sccontext.NewParameterContext(sccontext.TransactionType, net, tx)
 	sign := w.Accounts[0].SignHashable(net, tx)
 	_ = scCtx.AddSignature(w.Accounts[0].ScriptHash(), w.Accounts[0].Contract, w.Accounts[0].PublicKey(), sign)
 

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)