[ENHANCEMENT] Convert dedup pushdown to `composite + top_hits`

[LantaoJin]

Is your feature request related to a problem?
#3972 converts dedup command to collapse search when pushdown enabled. By benchmarking, the performance of collapse search shows worse than composite + top_hits or terms + top_hits. Here is the benchmarking result:

1. terms + top_hits 2000ms
2. composite + top_hits (missing_bucket false/true:) 3500ms/4500ms
3. collapse 13000ms

Additional, (1) and (2) can support dedup on bool, date etc fields and script expr, (3) only works on keyword/numeric field. and (2) can support search_after in future

What solution would you like?
Refactor to approach (2)

Do you have any additional context?

curl -XPOST "http://localhost:9200/big5/_search" -H 'Content-Type: application/json' -d'
{
  "size": 0,
  "_source": false,
  "aggs": {
    "packets": {
      "terms": {
        "field": "cloud.region",
        "size": 10000
      },
      "aggs": {
        "topN": {
          "top_hits": {
            "size": 1,
            "_source": true
          }
        }
      }
    }
  }
}'

2000ms

curl -XPOST "http://localhost:9200/big5/_search" -H 'Content-Type: application/json' -d'
{
  "size": 0,
  "timeout": "1m",
  "_source": false,
  "aggs": {
    "packets": {
      "composite": {
        "size": 10000,
        "sources": [
          {
            "name": {
              "terms": {
                "field": "cloud.region",
                "missing_bucket": true
              }
            }
          }
        ]
      },
      "aggs": {
        "topN": {
          "top_hits": {
            "size": 1,
            "_source": true
          }
        }
      }
    }
  }
}'

missing_bucket false 3500ms
missing_bucket true 4500ms

curl -XPOST "http://localhost:9200/big5/_search" -H 'Content-Type: application/json' -d'
{
  "from": 0,
  "size": 10000,
  "timeout": "1m",
  "_source": true,
  "collapse": {
    "field": "cloud.region"
  }
}'

13000ms

[anasalkouz]

@LantaoJin can we target 3.4 for this?