PartDesign: Allow 3D datums other than binders in bodies

### Problem description

Currently there are 2 ways to get a 3D datum object into a body:
1. Create a binder that references the object and put that into the body.
2. Add 3D geometry to a Part::Part2DObject (or Part::Part2DObjectPython) object and put that into the body.

The first option results in an additional object that just clutters the Tree View. The second option relies on a loophole in the PD code (which assumes that such an object only contains geometry on the XY-plane).

Without these steps the object is added as a base feature.

Forum topic:
https://forum.freecad.org/viewtopic.php?t=101221

### Workbench affected?

Part Design

### Steps to reproduce

n/a

### Expected behavior

When dragging a 3D object into a body it should be possible the specify that it should be treated as a datum object and not a base feature.

### Actual behavior

n/a

### Development version About Info (in Safe Mode)

```shell
<= v1.2
```

### Last known good version (optional)

```shell

```