### 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
return createShardsAvailabilityIndicatorService(ClusterState.EMPTY_STATE, Collections.emptyMap());
    }
    private static ShardsAvailabilityHealthIndicatorService createShardsAvailabilityIndicatorService(ClusterState clusterState) {
        return createShardsAvailabilityIndicatorService(clusterState, Collections.emptyMap());
    }
    private static ShardsAvailabilityHealthIndicatorService createShardsAvailabilityIndicatorService(
        ClusterState clusterState,
        final Map<ShardRoutingKey, ShardAllocationDecision> decisions
    ) {
        return createShardsAvailabilityIndicatorService(Settings.EMPTY, clusterState, decisions);
    }
    private static ShardsAvailabilityHealthIndicatorService createShardsAvailabilityIndicatorService(
        Settings nodeSettings,
        ClusterState clusterState,
        final Map<ShardRoutingKey, ShardAllocationDecision> decisions
    ) {
        var clusterService = mock(ClusterService.class);
        when(clusterService.state()).thenReturn(clusterState);
        var clusterSettings = new ClusterSettings(nodeSettings, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS);
        when(clusterService.getClusterSettings()).thenReturn(clusterSettings);
        var allocationService = mock(AllocationService.class);
        when(allocationService.explainShardAllocation(any(), any())).thenAnswer((Answer<ShardAllocationDecision>) invocation -> {
            ShardRouting shardRouting = invocation.getArgument(0);
            var key = new ShardRoutingKey(shardRouting.getIndexName(), shardRouting.getId(), shardRouting.primary());
            return decisions.getOrDefault(key, ShardAllocationDecision.NOT_TAKEN);
        });
        return new ShardsAvailabilityHealthIndicatorService(clusterService, allocationService);
    }
}