### 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. 

server/src/main/java/org/elasticsearch/TransportVersion.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 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;
import org.elasticsearch.core.Assertions;
import org.elasticsearch.internal.VersionExtension;
import java.io.IOException;
import java.lang.reflect.Field;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.NavigableMap;
import java.util.Set;
import java.util.TreeMap;
/**
 * Represents the version of the wire protocol used to communicate between a pair of ES nodes.
 * <p>
 * Prior to 8.8.0, the release {@link Version} was used everywhere. This class separates the wire protocol version from the release version.
 * <p>
 * Each transport version constant has an id number, which for versions prior to 8.9.0 is the same as the release version for backwards