text: Title: Serialized relationship id should be string
  Body: According to [JSON:API](https://jsonapi.org/format/#document-resource-object-identification), the `id` field should always be a string, but this isn't the case when fetching relationships through `SqlalchemyDataLayer`.
  
  I suspect the problem originates here: https://github.com/miLibris/flask-rest-jsonapi/blob/b4cb5576b75ffaf6463abb8952edc8839b03463f/flask_rest_jsonapi/data_layers/alchemy.py#L256
  
  @akira-dev I would be happy to contribute with a pull request if you consider this correct.