Sketcher: Can't create an external geometry from a linked sketch belonging to another document

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Problem description

Setup is simple:
* first file with a sketch
* second file with a link to above sketch
* sketch in second file tries to create external geometry from a linked sketch, but fails

```
16:51:20 pyException: Traceback (most recent call last):
File "", line 1, in
: Datum feature type is not yet supported as external geometry for a sketch
16:51:20 Test#Sketch: App.getDocument('Test').getObject('Sketch').addExternal("Link","Edge4")
16:51:20 Test#Sketch: Failed to add external geometry
```

Demo:

https://github.com/user-attachments/assets/acd862c9-e317-4239-a961-89fa758e8ba0

Files used in demo:
[LinkedSketchExternalGeometry.zip](https://github.com/user-attachments/files/16403945/LinkedSketchExternalGeometry.zip)

### Full version info

```shell
OS: Ubuntu 24.04 LTS (Custom/custom)
Word size of FreeCAD: 64-bit
Version: 0.22.0dev.38334 (Git) AppImage
Build type: Release
Branch: main
Hash: 61c964caa9a7e5bbc7a8d5f7d8914505d365113e
Python 3.11.9, Qt 5.15.13, Coin 4.0.2, Vtk 9.2.6, OCC 7.7.2
Locale: C/Default (C) [ OS: Ukrainian/Ukraine (uk_UA) ]
Installed mods:
* OpenTheme 2024.7.3
* fasteners 0.5.25
* parts_library
* Curves 0.6.42
```

### Subproject(s) affected?

Sketcher

### Anything else?

The only mention of this that I found is in https://forum.freecad.org/viewtopic.php?style=10&t=72596, but I'm not in PartDesign, these are simple sketches at the root of the document.

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct