[RFE] Add external skeleton support to PhysicalBone3D

[michael-nischt]

BoneAttachment3D have a super nice way refer any Skeleton:

It would be great to have the same functionality in the PhysicalBone3D.

Our use-case is that we prefer nesting scenes over inheriting scenes.
However, the lack of this feature forces to create an inherited scene to add rag-doll nodes.

Bone-attachments work just fine since we can specify an external skeleton.
We only need to briefly enable 'Editable Children` on the nested scene and to select the skeleton.

P.S. I guess the same applies PhysicalBone2D?