[BUG] ScoreFunction: variants are optional

[lihuimingxs]

What is the bug?

ScoreFunction: variants are optional. But now, it is required.

public class FunctionScore implements TaggedUnion<FunctionScore.Kind, Object>, JsonpSerializable {
        ...
	private final Kind _kind;
	private final Object _value;
        ...
}

This issue may result in some query scenarios not being implemented on the client side, like this:

GET /_search
{
  "query": {
    "function_score": {
      "query": {
        "match_all": {}
      },
      "boost": "5",
      "functions": [
        {
          "filter": {
            "match": {
              "test": "cat"
            }
          },
          "weight": 42
        }
      ],
      "max_boost": 42,
      "score_mode": "max",
      "boost_mode": "multiply",
      "min_score": 42
    }
  }
}

How can one reproduce the bug?

Attempt to build FunctionScore without Kind using Java client.

What is the expected behavior?

ScoreFunction: variants are optional.

What is your host/environment?

macOS 11.6 (Apple M1)

OpenSearch 2.9.0

Do you have any screenshots?

If applicable, add screenshots to help explain your problem.

Do you have any additional context?

Add any other context about the problem.

[lihuimingxs]

I will resolve it.