template-export needs .label suffix for enum fields

[tomwwinter]

Describe the bug
When exporting a dataset with the template export feature, its required to set the .label suffix on enum fields.
e.g. instad of {d.anrede} it must be {d.anrede.label}

Reason for that is, that the payload contains a nested object, instead the label of the value:

"anrede": {
    "id": "Herr",
    "label": "Herr"
}

To Reproduce
Steps to reproduce the behavior:

1. Go to 'any client'
2. Click on 'Datei generieren'
3. Select prepared Template
4. See error in downloaded file

Expected behavior
Should not be necessary to add .label, instead the label value should be send to the API directly.

Additional context
Add any other context about the problem here.

[sleidig]

To "future-proof" this for further extensions (maybe sending multiple entities or some calculated values to the pdf generator), I would suggest to put this logic into the TemplateExportService.
We already have some data transformation logic for the csv exports (see DownloadService and the formatting logic called there) which would probably be suitable for the data of pdf exports here as well?