make expensive metadata requestable with "include" key

[qwesda]

depends on #2

the metadata key in the api response should by default only contain information, that is available without any additional queries/processing.

expensive metadata should be provided if requested.

{
  "columns": [ ... ],
  "limit": 10,
  "order_by": [ ["total", "DESC"] ],
  "where": [],
  "offset_id": 8346,
  "offset": 0,
  "include" : [ 
    "count", // number of items
    "omitted", // number of items omitted due to order parameters (implicit NOT NULL)
    "prev", // url with offset_id for preceding chunk
  ]
}