### Description

Currently async_search get or status response include the following info about async_search:

```
  "is_partial" : true, 
  "is_running" : true, 
  "start_time_in_millis" : 1583945890986,
  "expiration_time_in_millis" : 1584377890986,
```

In Kibana we would also like to have something like `completion_time` that would indicate time when the search was completed. It doesn't have to be precise, having some margin of error would be fine. 

**Some context:**

Currently Kibana uses background tasks to monitor completion of searches and it saves the completion time and then later shows it to users when they restore the search. Now we are simplifying the implementation of searches restoration on Kibana side and removing search monitoring in background tasks [(RFC here)](https://docs.google.com/document/d/1xpvW0xHmdm6w6EyYzYd01JLhiPFWF1Ij-tcJxCRLO7U/edit#) so Kibana can't longer track on its side the completion time. 

x-pack/plugin/async-search/src/test/java/org/elasticsearch/xpack/search/AsyncSearchResponseTests.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.search;
import org.elasticsearch.TransportVersion;
import org.elasticsearch.action.search.SearchResponse;
import org.elasticsearch.action.search.ShardSearchFailure;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
import org.elasticsearch.common.io.stream.Writeable;