Create a new relationship between two entities

Example request

{
  "rel1": {
    "type": "belongsTo",
    "field": "id_users",
    "reference": {
      "entity": "users",
      "field": "id_users"
    }
  },
  "rel2": {
    "type": "hasMany",
    "reference": {
      "entity": "todos",
      "field": "id_users"
    }
  }
}