### 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/cluster/routing/allocation/ShardsAvailabilityHealthIndicatorService.java
Integer clusterShardsPerNode = clusterService.getClusterSettings().get(CLUSTER_TOTAL_SHARDS_PER_NODE_SETTING);
            Integer indexShardsPerNode = INDEX_TOTAL_SHARDS_PER_NODE_SETTING.get(indexMetadata.getSettings());
            assert (clusterShardsPerNode > 0 || indexShardsPerNode > 0) : "shards per node must exist if allocation decision is NO";
            
            boolean clusterShardsPerNodeShouldChange = false;
            if (clusterShardsPerNode > 0) {
                int minShardCountInTier = dataTierRoutingNodes.stream()
                    .map(RoutingNode::numberOfOwningShards)
                    .min(Integer::compareTo)
                    .orElse(-1);
                clusterShardsPerNodeShouldChange = minShardCountInTier >= clusterShardsPerNode;
            }
            boolean indexShardsPerNodeShouldChange = false;
            if (indexShardsPerNode > 0) {
                int minShardCountInTier = dataTierRoutingNodes.stream()
                    .map(routingNode -> routingNode.numberOfOwningShardsForIndex(indexMetadata.getIndex()))
                    .min(Integer::compareTo)
                    .orElse(-1);
                indexShardsPerNodeShouldChange = minShardCountInTier >= indexShardsPerNode;
            }
            
            if (preferredTier != null) {
                
                if (clusterShardsPerNodeShouldChange) {
                    Optional.ofNullable(ACTION_INCREASE_SHARD_LIMIT_CLUSTER_SETTING_LOOKUP.get(preferredTier))
                        .ifPresent(diagnosisDefs::add);
                }
                if (indexShardsPerNodeShouldChange) {
                    Optional.ofNullable(ACTION_INCREASE_SHARD_LIMIT_INDEX_SETTING_LOOKUP.get(preferredTier)).ifPresent(diagnosisDefs::add);
                }