### Description

Elasticsearch defines the concept of a [feature](https://www.elastic.co/guide/en/elasticsearch/reference/current/get-features-api.html) for the purpose of better managing them when snapshotting/restoring them via the [feature_states field](https://www.elastic.co/guide/en/elasticsearch/reference/current/create-snapshot-api.html#create-snapshot-api-feature-states)

The `shards_availability` health indicator currently diagnoses the condition of indices needing to be [restored from a snapshot](https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/cluster/routing/allocation/ShardsAvailabilityHealthIndicatorService.java#L138) and recommends a [troubleshooting guide](http://ela.st/restore-snapshot)

However, since `8.x` the restore API **must** use `feature_states` instead of specifying systems indices or system data streams as part of the `indices` field.

This proposes the `affected_resources` for the `restore_from_snapshot` diagnosis return the indices that are not part of a `feature` under the `INDICES` type, and a new type `FEATURES`.
e.g. instead of current behaviour
```
"affected_resources": {
  "indices": [
    ".geoip_databases",
    "test_index"
    ]
}
```
we could return:
```
"affected_resources": {
  "indices": [
    "test_index"
   ], 
  "features": [
     "geoip"
  ]
}
```



server/src/main/java/org/elasticsearch/health/Diagnosis.java
}
    }
    @Override
    public Iterator<? extends ToXContent> toXContentChunked(ToXContent.Params outerParams) {
        final Iterator<? extends ToXContent> resourcesIterator;
        if (affectedResources == null) {
            resourcesIterator = Collections.emptyIterator();
        } else {
            resourcesIterator = Iterators.flatMap(affectedResources.iterator(), s -> s.toXContentChunked(outerParams));
        }
        return Iterators.concat(Iterators.single((ToXContent) (builder, params) -> {
            builder.startObject();
            builder.field("id", definition.getUniqueId());
            builder.field("cause", definition.cause);
            builder.field("action", definition.action);
            builder.field("help_url", definition.helpURL);
            if (affectedResources != null && affectedResources.size() > 0) {
                builder.startObject("affected_resources");
            }
            return builder;
        }), resourcesIterator, Iterators.single((builder, params) -> {
            if (affectedResources != null && affectedResources.size() > 0) {
                builder.endObject();
            }
            builder.endObject();
            return builder;
        }));
    }
}