Add formatting to key-references in resource-files

### Description

We would like to be able to format values from a data field directly in the resource-files. This would give a simple way to get f.ex. thousand separator or comma when showing a numeric value, instead of having to concat several texts each time we need formatted text in the middle of a sentence. This would also make translation easier.

I imagine this could look something like this:

```
{
"id": "lang.info.fra.prefill",
"value": "Ved forrige innrapportering oppga dere å ha dyrket totalt {0} kvadratmeter bambus.",
"variables": [
{
"key": "Prefill.kvadratmeterbambus",
"dataSource": "dataModel.A3_RA-0775_M",
"formatting": {
"number": {
"decimalScale": 0,
"thousandSeparator": " "
}
}
}
]
}
```

Formatting f.ex. dates in a similar fashion would also be useful.

### Additional Information

Related: [https://github.com/Altinn/app-frontend-react/issues/1305](url)

Start by tracing how key references are handled in resource-files, then review the related app-frontend-react issue #1305. Done means values from a data field can receive the requested number formatting in a resource-file reference; date formatting is also identified as a related consideration.

Written by the indexing model from the issue text.