[#326] morph/balance: Provide interface to "transferX" method's invocations

Implement TransferX method on Balance contract client. Implement TransferX
method on Balance client's wrapper that encapsulates it's TransferX call.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/morph/client/balance/transfer.go

@@ -0,0 +1,53 @@
+package balance
+
+import (
+	"github.com/pkg/errors"
+)
+
+// TransferXArgs groups the arguments
+// of "transferX" invocation call.
+type TransferXArgs struct {
+	amount int64 // amount in GASe-12
+
+	sender []byte // sender's wallet script hash
+
+	recipient []byte // recipient's wallet script hash
+
+	details []byte // transfer details
+}
+
+// SetAmount sets amount of funds to transfer
+// in GASe-12.
+func (t *TransferXArgs) SetAmount(v int64) {
+	t.amount = v
+}
+
+// SetSender sets wallet script hash
+// of the sender of funds in a binary format.
+func (t *TransferXArgs) SetSender(v []byte) {
+	t.sender = v
+}
+
+// SetRecipient sets wallet script hash
+// of the recipient of funds in a binary format.
+func (t *TransferXArgs) SetRecipient(v []byte) {
+	t.recipient = v
+}
+
+// SetDetails sets details of the money transaction
+// in a binary format.
+func (t *TransferXArgs) SetDetails(v []byte) {
+	t.details = v
+}
+
+// TransferX invokes the call of "transferX" method
+// of NeoFS Balance contract.
+func (c *Client) TransferX(args TransferXArgs) error {
+	return errors.Wrapf(c.client.Invoke(
+		c.transferXMethod,
+		args.sender,
+		args.recipient,
+		args.amount,
+		args.details,
+	), "could not invoke method (%s)", c.transferXMethod)
+}