[#47] object: Specify ec_parent attribute for Object.Search

* Add $Object:ec.parent attribute to Object.Search specification.
* Update docs for Object.

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

object/service.proto

@@ -583,6 +583,9 @@ message SearchRequest {
     //   object_id of parent
     // * $Object:split.splitID \
     //   16 byte UUIDv4 used to identify the split object hierarchy parts
+    // * $Object:ec.parent \
+    //   If the object is stored according to EC policy, then ec_parent attribute
+    //   is set to return an id list of all related EC chunks.
     //
     // There are some well-known filter aliases to match objects by certain
     // properties:

proto-docs/object.md

@@ -875,6 +875,9 @@ prefix to the name. Here is the list of fields available via this prefix:
   object_id of parent
 * $Object:split.splitID \
   16 byte UUIDv4 used to identify the split object hierarchy parts
+* $Object:ec.parent \
+  If the object is stored according to EC policy, then ec_parent attribute
+  is set to return an id list of all related EC chunks.
 
 There are some well-known filter aliases to match objects by certain
 properties: