### 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/health/Diagnosis.java
/**
     * Represents a type of affected resource, together with the resources/abstractions that
     * are affected.
     */
    public static class Resource implements ChunkedToXContent {
        public static final String ID_FIELD = "id";
        public static final String NAME_FIELD = "name";
        public enum Type {
            INDEX("indices"),
            NODE("nodes"),
            SLM_POLICY("slm_policies"),
            SNAPSHOT_REPOSITORY("snapshot_repositories");
            private final String displayValue;
            Type(String displayValue) {
                this.displayValue = displayValue;
            }
        }
        private final Type type;
        @Nullable
        private Collection<String> values;
        @Nullable
        private Collection<DiscoveryNode> nodes;
        public Resource(Type type, Collection<String> values) {
            if (type == Type.NODE) {
                throw new IllegalArgumentException("Nodes should be modelled using the dedicated constructor");
            }
            this.type = type;
            this.values = values;
        }
        public Resource(Collection<DiscoveryNode> nodes) {