Add `impact_areas` field to impacts

eg
```
"impacts": [
            {
              "severity": 3,
              "description": "Searches might return slower than usual. Fewer redundant copies of the data exist on 2 indices [.ds-logslogslogs-2022.03.29-000001, my-index-000001].",
              "impact_areas": [ "search" ]
            }
          ]
```