Allow single function in FunctionScoreQuery.functions

[pquentin]

Note that this is not about supporting the shortcut form that we decided that we wouldn't support: #2852 (comment). This is just allowing the verbose form to be slightly smaller when there's a single function. And it's not breaking since X[] and X | X[] are equivalent for typed clients.

This allows turning this:

"function_score": {
  "functions": [
    {
      "random_score": {}
    }
  ]
}

into this:

"function_score": {
  "functions": {
    "random_score": {}
  }
}

which is nicer.

[github-actions]

Following you can find the validation changes against the target branch for the API.

API	Status	Request	Response
search	🔴	2640/2659 → 2646/2659	2659/2659

You can validate this API yourself by using the make validate target.