### 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/core/src/main/java/org/elasticsearch/xpack/core/search/action/AsyncStatusResponse.java
return successfulShards;
    }
    /**
     * Returns the number of skipped shards due to pre-filtering.
     */
    public int getSkippedShards() {
        return skippedShards;
    }
    /**
     * Returns the number of failed shards the search was executed on.
     */
    public int getFailedShards() {
        return failedShards;
    }
    /**
     * For a completed async search returns the completion status.
     * For a still running async search returns {@code null}.
     */
    public RestStatus getCompletionStatus() {
        return completionStatus;
    }
    /**
     * @return For CCS, clusters object that has information about the clustes being searched, such as total count
     * successful count and skipped. Will be null for local-only searches.
     */
    public SearchResponse.Clusters getClusters() {
        return clusters;
    }
}