### 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
return RED;
            } else if (replicas.available == false) {
                return YELLOW;
            } else {
                return GREEN;
            }
        }
        public String getSymptom() {
            var builder = new StringBuilder("This cluster has ");
            if (primaries.unassigned > 0
                || primaries.unassigned_new > 0
                || primaries.unassigned_restarting > 0
                || replicas.unassigned > 0
                || replicas.unassigned_restarting > 0) {
                builder.append(
                    Stream.of(
                        createMessage(primaries.unassigned, "unavailable primary shard", "unavailable primary shards"),
                        createMessage(primaries.unassigned_new, "creating primary shard", "creating primary shards"),
                        createMessage(primaries.unassigned_restarting, "restarting primary shard", "restarting primary shards"),
                        createMessage(replicas.unassigned, "unavailable replica shard", "unavailable replica shards"),
                        createMessage(replicas.unassigned_restarting, "restarting replica shard", "restarting replica shards")
                    ).flatMap(Function.identity()).collect(joining(", "))
                ).append(".");
            } else {
                builder.append("all shards available.");
            }
            return builder.toString();
        }
        private static Stream<String> createMessage(int count, String singular, String plural) {
            return switch (count) {