### 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
}
    /**
     * Generate a list of diagnoses that'll contain the instructions for a user to take to allow this shard to be assigned.
     * @param shardRouting An unassigned shard routing
     * @param state State of the cluster
     * @return A list of diagnoses for the provided unassigned shard
     */
    List<Diagnosis.Definition> diagnoseUnassignedShardRouting(ShardRouting shardRouting, ClusterState state) {
        List<Diagnosis.Definition> diagnosisDefs = new ArrayList<>();
        LOGGER.trace("Diagnosing unassigned shard [{}] due to reason [{}]", shardRouting.shardId(), shardRouting.unassignedInfo());
        switch (shardRouting.unassignedInfo().getLastAllocationStatus()) {
            case NO_VALID_SHARD_COPY:
                diagnosisDefs.add(ACTION_RESTORE_FROM_SNAPSHOT);
                break;
            case NO_ATTEMPT:
                if (shardRouting.unassignedInfo().isDelayed()) {
                    diagnosisDefs.add(DIAGNOSIS_WAIT_FOR_OR_FIX_DELAYED_SHARDS);
                } else {
                    diagnosisDefs.addAll(explainAllocationsAndDiagnoseDeciders(shardRouting, state));
                }
                break;
            case DECIDERS_NO:
                diagnosisDefs.addAll(explainAllocationsAndDiagnoseDeciders(shardRouting, state));
                break;
            case DELAYED_ALLOCATION:
                diagnosisDefs.add(DIAGNOSIS_WAIT_FOR_OR_FIX_DELAYED_SHARDS);
                break;
        }
        if (diagnosisDefs.isEmpty()) {
            diagnosisDefs.add(ACTION_CHECK_ALLOCATION_EXPLAIN_API);