### 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
var status = new ShardAllocationStatus(state.getMetadata());
        for (IndexRoutingTable indexShardRouting : state.routingTable()) {
            for (int i = 0; i < indexShardRouting.size(); i++) {
                IndexShardRoutingTable shardRouting = indexShardRouting.shard(i);
                status.addPrimary(shardRouting.primaryShard(), state, shutdown, verbose);
                for (ShardRouting replicaShard : shardRouting.replicaShards()) {
                    status.addReplica(replicaShard, state, shutdown, verbose);
                }
            }
        }
        return createIndicator(
            status.getStatus(),
            status.getSymptom(),
            status.getDetails(verbose),
            status.getImpacts(),
            status.getDiagnosis(verbose, maxAffectedResourcesCount)
        );
    }
    
    public static final String PRIMARY_UNASSIGNED_IMPACT_ID = "primary_unassigned";
    public static final String REPLICA_UNASSIGNED_IMPACT_ID = "replica_unassigned";
    public static final String RESTORE_FROM_SNAPSHOT_ACTION_GUIDE = "https://ela.st/restore-snapshot";
    public static final Diagnosis.Definition ACTION_RESTORE_FROM_SNAPSHOT = new Diagnosis.Definition(
        NAME,
        "restore_from_snapshot",
        "Elasticsearch isn't allowed to allocate some shards because there are no copies of the shards in the cluster. Elasticsearch will "
            + "allocate these shards when nodes holding good copies of the data join the cluster.",
        "If no such node is available, restore these indices from a recent snapshot.",
        RESTORE_FROM_SNAPSHOT_ACTION_GUIDE
    );