additional counters in _clusters response (#2301)

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

In `8.11` we are adding counters in the `_details` section of both async and 
synchronous search responses. The counters added are: `running`, `partial` 
and `failed`. elastic/elasticsearch#99566

Author: piergm

output/schema/schema.json

@@ -28,6 +28,9 @@ export class ClusterStatistics {
   skipped: integer
   successful: integer
   total: integer
+  running: integer
+  partial: integer
+  failed: integer
   details?: Dictionary<ClusterAlias, ClusterDetails>
 }