[#26] object: Define message for object header

In previous version of the format Object message included two header fields:
SystemHeader and list of ExtendedHeader. Due to the fact that all headers
were not encapsulated in a single message, some messages were forced to use
Object to convey the full header of the object (e.g. object.HeadResponse).
To resolve this semantic inconsistency, this commit defines a Header
message, consisting of a system and an extended portion. As a result, the
Object now consists of two fields - Header and Payload.

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

object/types.proto

@@ -109,13 +109,20 @@ message Transform {
     Type type  = 1;
 }
 
-message Object {
+// Header groups the information about the NeoFS object.
+message Header {
     // SystemHeader describes system header
     SystemHeader SystemHeader = 1;
+
     // ExtendedHeaders describes a set of an extended headers
     repeated ExtendedHeader ExtendedHeaders = 2;
+}
+
+message Object {
+    // Header carries the object header.
+    Header Header = 1;
     // Payload is an object's payload
-    bytes Payload             = 3;
+    bytes Payload = 2;
 }
 
 message PublicKey {

proto-docs/object.md

@@ -32,6 +32,7 @@
     - [Attribute](#object.Attribute)
     - [CreationPoint](#object.CreationPoint)
     - [ExtendedHeader](#object.ExtendedHeader)
+    - [Header](#object.Header)
     - [IntegrityHeader](#object.IntegrityHeader)
     - [Link](#object.Link)
     - [Object](#object.Object)
@@ -406,6 +407,18 @@ Attribute groups the parameters of the object attributes.
 | PublicKey | [PublicKey](#object.PublicKey) |  | PublicKey of owner of the object. Key is used for verification and can be based on NeoID or x509 cert. |
 
 
+<a name="object.Header"></a>
+
+### Message Header
+Header groups the information about the NeoFS object.
+
+
+| Field | Type | Label | Description |
+| ----- | ---- | ----- | ----------- |
+| SystemHeader | [SystemHeader](#object.SystemHeader) |  | SystemHeader describes system header |
+| ExtendedHeaders | [ExtendedHeader](#object.ExtendedHeader) | repeated | ExtendedHeaders describes a set of an extended headers |
+
+
 <a name="object.IntegrityHeader"></a>
 
 ### Message IntegrityHeader
@@ -438,8 +451,7 @@ Attribute groups the parameters of the object attributes.
 
 | Field | Type | Label | Description |
 | ----- | ---- | ----- | ----------- |
-| SystemHeader | [SystemHeader](#object.SystemHeader) |  | SystemHeader describes system header |
+| Header | [Header](#object.Header) |  | Header carries the object header. |
-| ExtendedHeaders | [ExtendedHeader](#object.ExtendedHeader) | repeated | ExtendedHeaders describes a set of an extended headers |
 | Payload | [bytes](#bytes) |  | Payload is an object's payload |