Add v2 version to go module name

Replace all elements from `v2` to root directory.

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

storagegroup/json.go

@@ -0,0 +1,14 @@
+package storagegroup
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/v2/rpc/message"
+	storagegroup "github.com/nspcc-dev/neofs-api-go/v2/storagegroup/grpc"
+)
+
+func (s *StorageGroup) MarshalJSON() ([]byte, error) {
+	return message.MarshalJSON(s)
+}
+
+func (s *StorageGroup) UnmarshalJSON(data []byte) error {
+	return message.UnmarshalJSON(s, data, new(storagegroup.StorageGroup))
+}