Pinned Retriever for Elasticsearch

This PR introduces a new pinned retriever for Elasticsearch, available under the search-business-rules plugin.

The pinned retriever allows users to explicitly pin specific documents (via ids or docs) so they always appear at the top of the search results, regardless of their organic relevance score. It is implemented as a compound retriever that rewrites internally to a PinnedQueryBuilder.

Key Features

• Pin Documents by ID or Full Doc: Pin specific documents either by their _id or by providing inline doc content.
• Custom Explanation Support: When used with "explain": true, the search response now includes a custom _explanation field for pinned results.
  Example:

  "description": "Pinned document by ids, original explanation:"
  

Example API Request

POST my-index/_search
{
  "explain": true,
  "retriever": {
    "pinned": {
      "ids": ["1", "2"],
      "retriever": {
        "standard": {
          "query": {
            "match": {
              "title": "machine learning"
            }
          }
        }
      }
    }
  }
}

Sample Response

"_explanation": {
  "value": 1.7014124e+38,
  "description": "Pinned document, original explanation:",
  "details": [
    {
      "value": 1,
      "description": "doc [0] with an original score of [1.7014124E38] is at rank [1] from the following source queries.",
      "details": [
        {
          // additional explanation details...
        }
      ]
    }
  ]
}