support customisation of the "highlighting" query response element

The objective and use case behind the proposed changes is to be able to receive not the out-of-the-box highlighting map

{
  ...
  "highlighting" : {
    "MA147LL/A" : {
      "manu" : [
        "<em>Apple</em> Computer Inc."
      ]
    }
  }
}

as illustrated in https://lucene.apache.org/solr/guide/7_2/highlighting.html#highlighting-in-the-query-response but to be able to alternatively name and customise the highlighting element of the query response to (for example) be like this

{
  ...
  "custom_highlighting" : [
    {
      "id" : "MA147LL/A",
      "snippets" : {
        "manu" : [
          "<em>Apple</em> Computer Inc."
        ]
      }
    }
  ]
}

where the highlighting element itself is a list and where the keys of each list element are 'knowable' in advance i.e. they are not 'unknowable' document ids.