sdk: Define container identifier type

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

pkg/container/id_test.go

@@ -0,0 +1,27 @@
+package container
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestIDV2_0(t *testing.T) {
+	cid := new(ID)
+
+	checksum := [sha256.Size]byte{}
+
+	_, err := rand.Read(checksum[:])
+	require.NoError(t, err)
+
+	cid.SetSHA256(checksum)
+
+	cidV2 := cid.ToV2()
+
+	cid2, err := IDFromV2(cidV2)
+	require.NoError(t, err)
+
+	require.Equal(t, cid, cid2)
+}