Add undocumented as=_explain to inspect the filtered query

[mbklein]

Adds _explain as a valid response format to return the query with auth filters added instead of executing the query and returning results.

Example:

POST /search/works?as=_explain
Content-Type: application/json

{
  "query": {
    "neural": {
      "embedding": {
        "query_text": "baseball negatives",
        "k": 10
      }
    }
  }
}

{
  "models": "mbk-dev-dc-v2-work",
  "body": {
    "from": 0,
    "query": {
      "neural": {
        "embedding": {
          "filter": {
            "bool": {
              "filter": [
                {
                  "terms": {
                    "published": [
                      true
                    ]
                  }
                },
                {
                  "terms": {
                    "visibility": [
                      "Institution",
                      "Public"
                    ]
                  }
                }
              ]
            }
          },
          "k": 10,
          "model_id": "TJqjgZUBz6P6wl85sCAv",
          "query_text": "baseball negatives"
        }
      }
    },
    "size": 10,
    "track_total_hits": true
  }
}