Move `GeometrySet` and `GeometryComponent` and subclasses to the `blender::bke` namespace. This wasn't done earlier since these were one of the first C++ classes used throughout Blender, but now it is common. Also remove the now-unnecessary C-header, since all users of the geometry set header are now in C++. Pull Request: https://projects.blender.org/blender/blender/pulls/109020