This also adds support for the SLM stats endpoint to the high level rest client.

Retrieving a policy now looks like:

```json
{
  "daily-snapshots" : {
    "version": 1,
    "modified_date": "2019-04-23T01:30:00.000Z",
    "modified_date_millis": 1556048137314,
    "policy" : {
      "schedule": "0 30 1 * * ?",
      "name": "<daily-snap-{now/d}>",
      "repository": "my_repository",
      "config": {
        "indices": ["data-*", "important"],
        "ignore_unavailable": false,
        "include_global_state": false
      },
      "retention": {}
    },
    "stats": {
      "snapshots_taken": 0,
      "snapshots_failed": 0,
      "snapshots_deleted": 0,
      "snapshot_deletion_failures": 0
    },
    "next_execution": "2019-04-24T01:30:00.000Z",
    "next_execution_millis": 1556048160000
  }
}
```

Relates to elastic#43663