### 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
diagnoseUnassignedShardRouting(routing, state).forEach(
                                definition -> addDefinition(definition, routing.getIndexName())
                            );
                        }
                    }
                }
                case INITIALIZING -> initializing++;
                case STARTED -> started++;
                case RELOCATING -> relocating++;
            }
        }
        private void addDefinition(Diagnosis.Definition diagnosisDefinition, String indexName) {
            diagnosisDefinitions.computeIfAbsent(diagnosisDefinition, (k) -> new HashSet<>()).add(indexName);
        }
    }
    private static boolean isUnassignedDueToTimelyRestart(ShardRouting routing, NodesShutdownMetadata shutdowns) {
        var info = routing.unassignedInfo();
        if (info == null || info.getReason() != UnassignedInfo.Reason.NODE_RESTARTING) {
            return false;
        }
        var shutdown = shutdowns.getAllNodeMetadataMap().get(info.getLastAllocatedNodeId());
        if (shutdown == null || shutdown.getType() != SingleNodeShutdownMetadata.Type.RESTART) {
            return false;
        }
        var now = System.nanoTime();
        var restartingAllocationDelayExpiration = info.getUnassignedTimeInNanos() + shutdown.getAllocationDelay().nanos();
        return now <= restartingAllocationDelayExpiration;
    }
    private static boolean isUnassignedDueToNewInitialization(ShardRouting routing) {
        return routing.primary() && routing.active() == false && getInactivePrimaryHealth(routing) == ClusterHealthStatus.YELLOW;