### 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 diagnosisDefs;
    }
    /**
     * For a shard that is unassigned due to a DECIDERS_NO result, this will explain the allocation and attempt to generate
     * a list of diagnoses that should allow the shard to be assigned.
     * @param shardRouting The shard routing that is unassigned with a last status of DECIDERS_NO
     * @param state Current cluster state
     * @return A list of diagnoses for the provided unassigned shard
     */
    private List<Diagnosis.Definition> explainAllocationsAndDiagnoseDeciders(ShardRouting shardRouting, ClusterState state) {
        LOGGER.trace("Executing allocation explain on shard [{}]", shardRouting.shardId());
        RoutingAllocation allocation = new RoutingAllocation(
            allocationService.getAllocationDeciders(),
            state,
            ClusterInfo.EMPTY,
            SnapshotShardSizeInfo.EMPTY,
            System.nanoTime()
        );
        allocation.setDebugMode(RoutingAllocation.DebugMode.ON);
        ShardAllocationDecision shardAllocationDecision = allocationService.explainShardAllocation(shardRouting, allocation);
        AllocateUnassignedDecision allocateDecision = shardAllocationDecision.getAllocateDecision();
        if (LOGGER.isTraceEnabled()) {
            if (allocateDecision.isDecisionTaken()) {
                LOGGER.trace("[{}]: Allocation decision [{}]", shardRouting.shardId(), allocateDecision.getAllocationDecision());
            } else {
                LOGGER.trace("[{}]: Decision taken [false]", shardRouting.shardId());
            }
        }
        if (allocateDecision.isDecisionTaken() && AllocationDecision.NO == allocateDecision.getAllocationDecision()) {