Search Response has completion_time and _cluster/details (#2260)

Updates the spec for the response objects for `POST _async_search`, `GET _async_search/:id`,
`GET _async_search/status/:id` and `GET _search`

Two high level changes have happened in 8.10:

1) `completion_time` and `completion_time_in_millis` (async_search only) elastic/elasticsearch#97700

2) Adding `details` metadata to the `_clusters` section of both async and synchronous search responses elastic/elasticsearch#97731

Author: quux00