[#82] object: Introduce new fields for EC header

* Regenerate protobufs as frostfs-api introduced `parent_split_id` and
  `parent_split_index` fields.
* Fix marshaller and converter for EC header.
* Extend v2 type `ECHeader` with `ParentSplitID` and `ParentSplitIndex` fields.
* Fix message_test for `Object`. Also generate EC headers and check it.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

object/types.go

@@ -40,12 +40,14 @@ type SplitHeader struct {
 }
 
 type ECHeader struct {
-	Parent       *refs.ObjectID
-	Index        uint32
-	Total        uint32
-	Header       []byte
-	HeaderLength uint32
-	Signature    []byte
+	Parent           *refs.ObjectID
+	ParentSplitID    []byte
+	ParentSplitIndex uint32
+	Index            uint32
+	Total            uint32
+	Header           []byte
+	HeaderLength     uint32
+	Signature        []byte
 }
 
 type Header struct {