simplify multi-sheet responses

[tripodsan]

in order to remove unexpected, unused properties we should change the format (again):

single sheet response:

HTTP/2 200
content-type: application/json
x-helix-data-version: 4
x-helix-data-type: sheet
x-helix-data-names: sheet1

{
  "offset": 0,
  "limit": 100,
  "total": 200,
  "data": [ ... ]
}

multi sheet response:

HTTP/2 200
content-type: application/json
x-helix-data-version: 4
x-helix-data-type: multi-sheet
x-helix-data-names: sheet1, sheet2

{
  "sheets": {
    "sheet1": {
      "offset": 0,
      "limit": 100,
      "total": 200,
      "data": [
        ...
      ]
    },
    "sheet2": {
      "offset": 0,
      "limit": 100,
      "total": 200,
      "data": [
        ...
      ]
    }
  }
}

/cc @trieloff @davidnuescheler

[davidnuescheler]

Let's make sure we wait with this and time it until we have no more use of multiple sheets anymore in production which should be in a couple of weeks.

[tripodsan]

ok, so I use the old format in adobe/helix-json-filter#6