Query templates allow only "string" placeholders

[agazzarini]

That has been raised in a scenario where Elasticsearch is used as a search platform.
If we have a query template like this:

{
  "query": {
    "multi_match": {
      "query": "$query",
      "fields": ["title^10","overview"]
    }
  }
}

we cannot declare, as a placeholder, the whole fields array.
In that scenario, the change between v1.0 and v1.1 was about a different fields list, so the only chance was to create two (almost equal) query templates, each of one with a different fields list.