[#26] object: Define object split info message

This commit defines SplitHeader message that encapsulates data about
spawning the object through a payload splitting. Message fields contain
information about the hierarchy in the split-chain as well as information
about the origin object.

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

object/types.proto

@@ -100,6 +100,24 @@ message Transform {
     Type type  = 1;
 }
 
+// SplitHeader groups information about spawning the object through a payload splitting.
+message SplitHeader {
+    // Parent carries identifier of the origin object.
+    refs.ObjectID Parent = 1;
+
+    // Previous carries identifier of the left split neighbor.
+    refs.ObjectID Previous = 2;
+
+    // Previous carries identifier of the right split neighbor.
+    refs.ObjectID Next = 3;
+
+    // Children carries list of identifiers of the objects generated by splitting the current.
+    repeated refs.ObjectID Children = 4;
+
+    // Origin carries the header of the origin object.
+    Header Origin = 5;
+}
+
 // Header groups the information about the NeoFS object.
 message Header {
     // SystemHeader describes system header

proto-docs/object.md

@@ -36,6 +36,7 @@
     - [Link](#object.Link)
     - [Object](#object.Object)
     - [PublicKey](#object.PublicKey)
+    - [SplitHeader](#object.SplitHeader)
     - [SystemHeader](#object.SystemHeader)
     - [Tombstone](#object.Tombstone)
     - [Transform](#object.Transform)
@@ -453,6 +454,21 @@ Header groups the information about the NeoFS object.
 | Value | [bytes](#bytes) |  | Value contains marshaled ecdsa public key |
 
 
+<a name="object.SplitHeader"></a>
+
+### Message SplitHeader
+SplitHeader groups information about spawning the object through a payload splitting.
+
+
+| Field | Type | Label | Description |
+| ----- | ---- | ----- | ----------- |
+| Parent | [refs.ObjectID](#refs.ObjectID) |  | Parent carries identifier of the origin object. |
+| Previous | [refs.ObjectID](#refs.ObjectID) |  | Previous carries identifier of the left split neighbor. |
+| Next | [refs.ObjectID](#refs.ObjectID) |  | Previous carries identifier of the right split neighbor. |
+| Children | [refs.ObjectID](#refs.ObjectID) | repeated | Children carries list of identifiers of the objects generated by splitting the current. |
+| Origin | [Header](#object.Header) |  | Origin carries the header of the origin object. |
+
+
 <a name="object.SystemHeader"></a>
 
 ### Message SystemHeader