Adds rescore parameter to KNNQuery

[jmazanec15]

Description

Adds rescore parameter to knn query. With this commit, the rescore is a no-op. The functionality and validation will be added in a later commit.

The valid options look like:

"query": {
  "knn": {
    "target_field": {
      "vector": [0, 0],
      "k": 100,
      "rescore": true
    }
  }
}

"query": {
  "knn": {
    "target_field": {
      "vector": [0, 0],
      "k": 100,
      "rescore": {}
    }
  }
}

  "query": {
    "knn": {
      "target_field": {
        "vector": [0, 0],
        "k": 100,
        "rescore": {
            "oversample_factor": 1.2
        }
      }
    }
  }

Will update changelog once feature is completed

Related Issues

Related to #1779

Check List

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