[#83] client: Implement status library

Define base `Status` interface. Provide the functionality to distinguish
success and failure returns. Provide functionality to transport statuses
over NeoFS API V2 protocol. Support success `OK` and failure `INTERNAL`
returns.

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

client/status/common_test.go

@@ -0,0 +1,26 @@
+package apistatus_test
+
+import (
+	"testing"
+
+	apistatus "github.com/nspcc-dev/neofs-sdk-go/client/status"
+	"github.com/stretchr/testify/require"
+)
+
+func TestServerInternal_Message(t *testing.T) {
+	const msg = "some message"
+
+	var st apistatus.ServerInternal
+
+	res := st.Message()
+	resv2 := apistatus.ToStatusV2(st).Message()
+	require.Empty(t, res)
+	require.Empty(t, resv2)
+
+	st.SetMessage(msg)
+
+	res = st.Message()
+	resv2 = apistatus.ToStatusV2(st).Message()
+	require.Equal(t, msg, res)
+	require.Equal(t, msg, resv2)
+}