[#31] refs: Define ContainerID message

Define ContainerID message in refs package. In the future, this structure can be
ported into messages in place of the container identifier field to maintain
format uniformity. Also, storing the identifier in a dedicated message will
allow, if necessary, to expand it with additional information without losing
backward compatibility within one version of the API.

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

proto-docs/refs.md

@@ -7,6 +7,7 @@
 
   - Messages
     - [Address](#refs.Address)
+    - [ContainerID](#refs.ContainerID)
     - [ObjectID](#refs.ObjectID)
     
 
@@ -35,6 +36,17 @@ Address of object (container id + object id)
 | CID | [bytes](#bytes) |  | CID is container identifier |
 
 
+<a name="refs.ContainerID"></a>
+
+### Message ContainerID
+ContainerID groups information about the NeoFS container identifier.
+
+
+| Field | Type | Label | Description |
+| ----- | ---- | ----- | ----------- |
+| Value | [bytes](#bytes) |  | Value carries the container identifier in a binary format. |
+
+
 <a name="refs.ObjectID"></a>
 
 ### Message ObjectID

refs/types.proto

@@ -22,4 +22,10 @@ message Address {
 message ObjectID {
     // Value carries the object identifier in a binary format.
     bytes Value = 1;
-}
+}
+
+// ContainerID groups information about the NeoFS container identifier.
+message ContainerID {
+    // Value carries the container identifier in a binary format.
+    bytes Value = 1;
+}