### 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 HealthIndicatorDetails.EMPTY;
            }
        }
        public List<HealthIndicatorImpact> getImpacts() {
            final List<HealthIndicatorImpact> impacts = new ArrayList<>();
            if (primaries.indicesWithUnavailableShards.isEmpty() == false) {
                String impactDescription = String.format(
                    Locale.ROOT,
                    "Cannot add data to %d %s [%s]. Searches might return incomplete results.",
                    primaries.indicesWithUnavailableShards.size(),
                    primaries.indicesWithUnavailableShards.size() == 1 ? "index" : "indices",
                    getTruncatedIndices(primaries.indicesWithUnavailableShards, clusterMetadata)
                );
                impacts.add(
                    new HealthIndicatorImpact(
                        NAME,
                        PRIMARY_UNASSIGNED_IMPACT_ID,
                        1,
                        impactDescription,
                        List.of(ImpactArea.INGEST, ImpactArea.SEARCH)
                    )
                );
            }
            /*
             * It is possible that we're working with an intermediate cluster state, and that for an index we have no primary but a replica
             * that is reported as unavailable. That replica is likely being promoted to primary. The only impact that matters at this
             * point is the one above, which has already been reported for this index.
             */
            Set<String> indicesWithUnavailableReplicasOnly = new HashSet<>(replicas.indicesWithUnavailableShards);
            indicesWithUnavailableReplicasOnly.removeAll(primaries.indicesWithUnavailableShards);