[#198] sdk: Implement XHeader type

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

pkg/xheader_test.go

@@ -0,0 +1,25 @@
+package pkg
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestXHeader(t *testing.T) {
+	x := NewXHeader()
+
+	key := "some key"
+	val := "some value"
+
+	x.SetKey(key)
+	x.SetValue(val)
+
+	require.Equal(t, key, x.Key())
+	require.Equal(t, val, x.Value())
+
+	xV2 := x.ToV2()
+
+	require.Equal(t, key, xV2.GetKey())
+	require.Equal(t, val, xV2.GetValue())
+}