implement multivalue_doc_count aggregation

[chamchi00800]

Description

implement multivalue_doc_count aggregation.
The multivalue_doc_count function returns the number of documents that have two or more values.

1. Implemented with reference to the mentioned value_count.
2. exlclude precomputed logic in this PR.
   exclude precomputeLearUsingStarTree method because it is not registered in starTree,
   and the PR seems to be getting too huge.

Related Issues

Resolves #20329

test

(1) Unit test : I wrote a test for the aggregator in MultiValueDocCountAggregatorTest

(2) Run locally and test with real curl

step1 create index and insert documents

curl -X PUT "localhost:9200/products?pretty" -H 'Content-Type: application/json' -d'
{
  "mappings": {
    "properties": {
      "categories": {
        "type": "keyword"
      }
    }
  }
}'

curl -X POST "localhost:9200/products/_bulk?refresh=true&pretty" -H 'Content-Type: application/json' -d'
{"index": {"_id": "1"}}
{"categories": ["Electronics", "Computers"]}
{"index": {"_id": "2"}}
{"categories": ["Books"]}
{"index": {"_id": "3"}}
{"categories": ["Electronics", "Phones", "Accessories"]}
{"index": {"_id": "4"}}
{"categories": ["Clothing"]}
'

step2 aggregation by multivalue_doc_count

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[github-actions]

❌ Gradle check result for a1c973e: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for f60f066: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?