enhance _features by adding indices in the response

[TomonoriSoejima]

Description

Currently a response from GET _features/ does not include associated indices

{
  "features": [
    {
      "name": "inference_plugin",
      "description": "Inference plugin for managing inference services and inference"
    },
    {
      "name": "enrich",
      "description": "Manages data related to Enrich policies"
    },
    {
      "name": "fleet",
      "description": "Manages configuration for Fleet"
    }
  ]
}

It would be great if the response can include a matching indexes as it is done in snapshot creation time.

https://github.com/elastic/elasticsearch/blob/55cf37cedd81b2b452f3ca2240ec78b7374e089d/server/src/main/java/org/elasticsearch/snapshots/SnapshotsService.java#L3768-L3799

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[pxsalehi]

What is the use case for knowing the list of indices in a feature? My understanding is that the feature itself should know which matching indices to snapshot/restore, i.e., this is internal to the feature.

[TomonoriSoejima]

In my users's particular case, knowing what indices get included allows them to do proper sizing for the disk required for this snapshot.

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[ywangd]

I am routing this to core-infra since the GetFeatures API is owned by them.