Construct endpoints with chained datasources

[adamkdean]

It would be great if we could construct endpoints using values from chained datasources in the same way that we construct filters.

{
  "datasource": {
    "key": "models",
    "source": {
      "endpoint": "api/models/{makeId}"
    },
    "chained": {
      "datasource": "makes",
      "outputParam": {
        "param": "results.0._id",
        "field": "makeId"
      }
    }
  }
}

[jimlambie]

Closed via #297