Support the 'fields' option in inner_hits and top_hits.

[jtibshirani]

This PR adds support for the 'fields' option in the following places:

• Anytime inner_hits is used, for both fetching nested/ child docs and field collapsing
• The top_hits aggregation

Addresses #61949.

[jtibshirani]

Here's an example mapping and search request.

PUT test
{
  "mappings": {
    "properties": {
      "object": {
        "type": "nested",
        "properties": {
          "field": {
            "type": "keyword"
          }
        }
      }
    }
  }
}

POST test/_search
{
  "query": {
    "nested": {
      "query": { "match_all": {} },
      "path": "field",
      "inner_hits": {
        "_source": false,
        "fields": ["object.field"]
      }
    }
  }
}

Note that the full path to the field must be specified (object.field), which matches the behavior for docvalue_fields.

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[jimczi]

LGTM