Add fields to document get response to support stored_fields param

[boybundit]

Description

Add fields to document get response to support stored_fields param

Issues Resolved

To support getting document with stored_fields param

PUT /my-index
{
  "mappings": {
    "properties": {
      "demo": {
        "type" : "keyword",
        "store": true
      }
    }
  }
}

GET /my-index/_doc/1?stored_fields=foo-stored

{
    "_index": "my-index",
    "_id": "1",
    "_version": 1,
    "_seq_no": 0,
    "_primary_term": 1,
    "found": true,
    "fields": {
        "foo-stored": [
            "bar"
        ]
    }
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.23%. Comparing base (06a6dc8) to head (a86c9e8).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #526      +/-   ##
==========================================
+ Coverage   57.29%   66.23%   +8.93%     
==========================================
  Files         315      364      +49     
  Lines        9823     8585    -1238     
==========================================
+ Hits         5628     5686      +58     
+ Misses       2902     1588    -1314     
- Partials     1293     1311      +18     

Flag	Coverage Δ
integration	59.56% <ø> (+8.72%)	⬆️
unit	81.64% <ø> (+68.79%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
opensearchapi/api_document-get.go	100.00% <ø> (ø)

... and 250 files with indirect coverage changes