### 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/test/java/org/elasticsearch/cluster/routing/allocation/ShardsAvailabilityHealthIndicatorServiceTests.java
private static IndexRoutingTable index(IndexMetadata indexMetadata, ShardAllocation primaryState, ShardAllocation... replicaStates) {
        var index = indexMetadata.getIndex();
        var shardId = new ShardId(index, 0);
        var builder = IndexRoutingTable.builder(index);
        builder.addShard(createShardRouting(shardId, true, primaryState));
        for (var replicaState : replicaStates) {
            builder.addShard(createShardRouting(shardId, false, replicaState));
        }
        return builder.build();
    }
    private static ShardRouting createShardRouting(ShardId shardId, boolean primary, ShardAllocation allocation) {
        var routing = newUnassigned(
            shardId,
            primary,
            getSource(primary, allocation.state),
            new UnassignedInfo(UnassignedInfo.Reason.INDEX_CREATED, null)
        );
        if (allocation.state == INITIALIZING) {
            return routing;
        }
        routing = routing.initialize(allocation.nodeId, null, 0);
        routing = routing.moveToStarted(ShardRouting.UNAVAILABLE_EXPECTED_SHARD_SIZE);
        if (allocation.state == AVAILABLE) {
            return routing;
        }
        if (allocation.state == UNAVAILABLE) {
            return routing.moveToUnassigned(Optional.ofNullable(allocation.unassignedInfo).orElse(randomFrom(nodeLeft(), decidersNo())));
        }
        if (allocation.state == RESTARTING) {
            return routing.moveToUnassigned(