[#172] v2/container: Add JSON converter for container

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

v2/container/json.go

@@ -0,0 +1,36 @@
+package container
+
+import (
+	"github.com/golang/protobuf/jsonpb"
+	container "github.com/nspcc-dev/neofs-api-go/v2/container/grpc"
+)
+
+func ContainerToJSON(c *Container) []byte {
+	if c == nil {
+		return nil
+	}
+
+	msg := ContainerToGRPCMessage(c)
+	m := jsonpb.Marshaler{}
+
+	s, err := m.MarshalToString(msg)
+	if err != nil {
+		return nil
+	}
+
+	return []byte(s)
+}
+
+func ContainerFromJSON(data []byte) *Container {
+	if len(data) == 0 {
+		return nil
+	}
+
+	msg := new(container.Container)
+
+	if err := jsonpb.UnmarshalString(string(data), msg); err != nil {
+		return nil
+	}
+
+	return ContainerFromGRPCMessage(msg)
+}