### 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/test/java/org/elasticsearch/cluster/routing/allocation/ShardsAvailabilityHealthIndicatorServiceTests.java
if (indexNameToPriorityMap != null) {
            for (Map.Entry<String, Integer> indexNameToPriority : indexNameToPriorityMap.entrySet()) {
                String indexName = indexNameToPriority.getKey();
                IndexMetadata.Builder indexMetadataBuilder = new IndexMetadata.Builder(indexName);
                Settings settings = Settings.builder()
                    .put(IndexMetadata.SETTING_PRIORITY, indexNameToPriority.getValue())
                    .put(IndexMetadata.INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), 1)
                    .put(IndexMetadata.INDEX_NUMBER_OF_REPLICAS_SETTING.getKey(), 1)
                    .put(IndexMetadata.SETTING_INDEX_VERSION_CREATED.getKey(), Version.CURRENT)
                    .build();
                indexMetadataBuilder.settings(settings);
                indexMetadataList.add(indexMetadataBuilder.build());
            }
        }
        return indexMetadataList;
    }
    private static ClusterState createClusterStateWith(
        List<IndexMetadata> indexMetadataList,
        List<IndexRoutingTable> indexRoutingTables,
        List<NodeShutdown> nodeShutdowns,
        List<DiscoveryNode> nodes
    ) {
        var routingTableBuilder = RoutingTable.builder();
        for (IndexRoutingTable indexRoutingTable : indexRoutingTables) {
            routingTableBuilder.add(indexRoutingTable);
        }
        var nodesShutdownMetadata = new NodesShutdownMetadata(
            nodeShutdowns.stream()
                .collect(
                    toMap(