Add support for search using the "fields" parameter with knn_vector field

[e-emoto]

Description

This change implements valueFetcher in KNNVectorFieldType so search can support using the "fields" parameter when used with the knn_vector field type. ITs for this change were added to OpenSearchIT after the change was tested manually.

Manual Testing Output

curl -X PUT "localhost:9200/test_index" -H 'Content-Type: application/json' -d' 
{
  "settings" : {
    "number_of_shards" : 1,
    "number_of_replicas" : 0,
    "index.knn": true
  },
  "mappings": {
       "properties": {
       "test_field1": {
           "type": "knn_vector",
           "dimension": 2,
           "method": {
               "name":"hnsw",
               "engine":"nmslib",
               "space_type": "l2",
               "parameters":{
                 "m":20,
                 "ef_construction": 23
               }
           }
      },"test_field2": {
           "type": "knn_vector",
           "dimension": 3,
           "method": {
               "name":"hnsw",
               "engine":"nmslib",
               "space_type": "l2",
               "parameters":{
                 "m":20,
                 "ef_construction": 23
               }
           }
      },"test_field3": {
           "type": "text"
      }
   }
  }
}
'

curl -X PUT "localhost:9200/_bulk" -H 'Content-Type: application/json' -d'
{ "index": { "_index": "test_index" } }
{ "test_field1": [1.5, 5.5], "test_field2": [1.5, 5.5, 3.5], "test_field3": "text 1"}
{ "index": { "_index": "test_index" } }
{ "test_field1": [2.5, 3.5], "test_field2": [2.5, 3.5, 7.5], "test_field3": "text 2"}
{ "index": { "_index": "test_index" } }
{ "test_field1": [4.5, 5.5], "test_field2": [4.5, 5.5, 1.5]}
{ "index": { "_index": "test_index" } }
{ "test_field1": [1.5, 5.5], "test_field2": [1.5, 5.5, 2.5]}
{ "index": { "_index": "test_index" } }
{ "test_field3": "text 3"}
{ "index": { "_index": "test_index" } }
{ "test_field3": "text 4"}
'

curl -X GET "localhost:9200/test_index/_search?pretty" -H 'Content-Type: application/json' -d'
{
  "fields": ["test_field1"],
  "query": {
    "knn": {
      "test_field1": {
        "vector": [2.0, 4.0],
        "k": 10
      }
    }
  }
}
'

{
  "took" : 4,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 4,
      "relation" : "eq"
    },
    "max_score" : 0.6666667,
    "hits" : [
      {
        "_index" : "test_index",
        "_id" : "6lOiwpMB0RzOL9-Fc3K5",
        "_score" : 0.6666667,
        "_source" : {
          "test_field1" : [
            2.5,
            3.5
          ],
          "test_field2" : [
            2.5,
            3.5,
            7.5
          ],
          "test_field3" : "text 2"
        },
        "fields" : {
          "test_field1" : [
            2.5,
            3.5
          ]
        }
      },
      {
        "_index" : "test_index",
        "_id" : "6VOiwpMB0RzOL9-Fc3K5",
        "_score" : 0.2857143,
        "_source" : {
          "test_field1" : [
            1.5,
            5.5
          ],
          "test_field2" : [
            1.5,
            5.5,
            3.5
          ],
          "test_field3" : "text 1"
        },
        "fields" : {
          "test_field1" : [
            1.5,
            5.5
          ]
        }
      },
      {
        "_index" : "test_index",
        "_id" : "7FOiwpMB0RzOL9-Fc3K5",
        "_score" : 0.2857143,
        "_source" : {
          "test_field1" : [
            1.5,
            5.5
          ],
          "test_field2" : [
            1.5,
            5.5,
            2.5
          ]
        },
        "fields" : {
          "test_field1" : [
            1.5,
            5.5
          ]
        }
      },
      {
        "_index" : "test_index",
        "_id" : "61OiwpMB0RzOL9-Fc3K5",
        "_score" : 0.10526316,
        "_source" : {
          "test_field1" : [
            4.5,
            5.5
          ],
          "test_field2" : [
            4.5,
            5.5,
            1.5
          ]
        },
        "fields" : {
          "test_field1" : [
            4.5,
            5.5
          ]
        }
      }
    ]
  }
}

Related Issues

Resolves #1633

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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.

[navneet1v]

@e-emoto please fix the failing CIs

[navneet1v]

ITs for this change were added to OpenSearchIT after the change was tested manually.

can you add the manual testing result on the PR?

[jmazanec15]

LGTM thanks @e-emoto !

[e-emoto]

Created backport PR manually to fix issue with sign-off email:
#2411