User-defined attributes for XLIFF translation files

### Describe the problem

My project has a backend and a frontend sharing the same set of translations managed by weblate.

Right now I have a whitelist of keys in the backend to determine which translations should be exported to the frontend. I don't want to expose every translation to the frontend because of the file size.

As this is rather tedious I would like to manage the "whitelist" via a label/setting/checkbox directly in weblate.
I'm currently using XLIFF files which would support user-defined attributes per `trans-unit` but there seems to be no way in weblate to set them.

### Describe the solution you'd like

There could maybe be a similar setting like "label" for each translation key which would then generate a `trans-unit` like

```

```

where `[content]` could be the value set in the UI (in my case f.e. `frontend` or something like that).

### Describe alternatives you've considered

Explanation, Labels, Flags

They seem to be stored in weblates database but not in the xliff-file.

### Screenshots

_No response_

### Additional context

Maybe there is another way to achieve this, however I haven't found it.