[#168] storageGroup: Implement binary/JSON encoders/decoders on SG

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

v2/storagegroup/json_test.go

@@ -0,0 +1,20 @@
+package storagegroup_test
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/storagegroup"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStorageGroupJSON(t *testing.T) {
+	sg := generateSG()
+
+	data, err := sg.MarshalJSON()
+	require.NoError(t, err)
+
+	sg2 := new(storagegroup.StorageGroup)
+	require.NoError(t, sg2.UnmarshalJSON(data))
+
+	require.Equal(t, sg, sg2)
+}