Is your feature request related to a problem? Please describe

It's is known that we can get value from DocValues. When the document has a high number of fields, and we just want to retrieve one/two field, DocValues performs better than stored fields.
https://sease.io/2020/03/docvalues-vs-stored-fields-apache-solr-features-and-performance-smackdown.html

But DocValues doesn't work in flat_object filed.

Example:

PUT /test-index/
{
  "mappings": {
    "properties": {
      "issue": {
        "type": "flat_object"
      }
    }
  }
}

PUT /test-index/_doc/1
{
  "issue": {
    "number": "123456",
    "labels": {
      "version": "2.1",
      "backport": [
        "2.0",
        "1.3"
      ],
      "category": {
        "type": "API",
        "level": "enhancement"
      }
    }
  }
}

GET test-index/_search
{
    "stored_fields": "_none_",
    "docvalue_fields":["issue.number"]
}

issue.number is not in the result.

Describe the solution you'd like

As we have store DocValues for flat_object filed, we should retrieve value from DocValues in flat_object filed when necessary. I'm pleasure to implement it.

Related component

Storage:Performance

Describe alternatives you've considered

No response

Additional context

No response