### 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
List<Diagnosis.Definition> diagnoseAllocationResults(
        ShardRouting shardRouting,
        ClusterState state,
        List<NodeAllocationResult> nodeAllocationResults
    ) {
        IndexMetadata index = state.metadata().index(shardRouting.index());
        List<Diagnosis.Definition> diagnosisDefs = new ArrayList<>();
        if (index != null) {
            diagnosisDefs.addAll(checkIsAllocationDisabled(index, nodeAllocationResults));
            diagnosisDefs.addAll(checkDataTierRelatedIssues(index, nodeAllocationResults, state));
        }
        if (diagnosisDefs.isEmpty()) {
            diagnosisDefs.add(ACTION_CHECK_ALLOCATION_EXPLAIN_API);
        }
        return diagnosisDefs;
    }
    /**
     * Convenience method for filtering node allocation results by decider outcomes.
     * @param deciderName The decider that is being checked
     * @param outcome The outcome expected
     * @return A predicate that returns true if the decision exists and matches the expected outcome, false otherwise.
     */
    private static Predicate<NodeAllocationResult> hasDeciderResult(String deciderName, Decision.Type outcome) {
        return (nodeResult) -> nodeResult.getCanAllocateDecision()
            .getDecisions()
            .stream()
            .anyMatch(decision -> deciderName.equals(decision.label()) && outcome == decision.type());
    }
    /**
     * Generates a list of diagnoses if a shard cannot be allocated anywhere because allocation is disabled for that shard