Add `BoneAttachment3D.get_external_skeleton_path`, deprecate `get_external_skeleton`

[Flynsarmy]

trafficstars

Resolves https://github.com/godotengine/godot-proposals/issues/10670

[AThousandShips]

Why only the get? Why not set_external_skeleton_path? This makes things more confusing

[Flynsarmy]

You're absolutely right! I'll update the setter too.

[fire]

@TokageItLab I think this is ok, but I may have missed a conceptual problem. Can you review?