### 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
new UnassignedInfo(
                    UnassignedInfo.Reason.NODE_RESTARTING,
                    null,
                    null,
                    -1,
                    allocation.unassignedTimeNanos != null ? allocation.unassignedTimeNanos : 0,
                    0,
                    false,
                    UnassignedInfo.AllocationStatus.DELAYED_ALLOCATION,
                    Set.of(),
                    allocation.nodeId
                )
            );
        }
        throw new AssertionError("Unexpected state [" + allocation.state + "]");
    }
    private static RecoverySource getSource(boolean primary, ShardState state) {
        if (primary) {
            return state == INITIALIZING
                ? RecoverySource.EmptyStoreRecoverySource.INSTANCE
                : RecoverySource.ExistingStoreRecoverySource.INSTANCE;
        } else {
            return RecoverySource.PeerRecoverySource.INSTANCE;
        }
    }
    public enum ShardState {
        UNAVAILABLE,
        INITIALIZING,
        AVAILABLE,
        RESTARTING