[#398] morph/client: Add method to read magic number

Implement `Client.MagicNumber` method that returns magic number of the
network to which the underlying RPC node client is connected.

Network magic value is received via `GetNetwork` method.

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

pkg/morph/client/client.go

@@ -166,3 +166,9 @@ func toStackParameter(value interface{}) (sc.Parameter, error) {
 
 	return result, nil
 }
+
+// MagicNumber returns the magic number of the network
+// to which the underlying RPC node client is connected.
+func (c *Client) MagicNumber() uint64 {
+	return uint64(c.client.GetNetwork())
+}