Nest foreign key relations

Serialize foreign key references with a nested object, e.g.:

{
  "name": "service-production",
  "owner": {
    "id": "5d8201b0..."
  },
  // ...
}

Instead of e.g.:

{
  "name": "service-production",
  "owner_id": "5d8201b0...",
  // ...
}

This approach makes it possible to inline more information about the related resource without having to change the structure of the response or introduce more top-level response fields, e.g.:

{
  "name": "service-production",
  "owner": {
    "id": "5d8201b0...",
    "name": "Alice",
    "email": "[email protected]"
  },
  // ...
}