Make it possible to copy a RaycastingScene

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### Proposed new feature or change

Right now, the `RaycastingScene` object contains a unique_ptr, which prevents making copies of it.

In my application I have to add the same objects to the scene each iteration. It would much more efficient if I deep copied a `RaycastingScene` object in memory.

See L253 here: https://www.open3d.org/html/cpp_api/_raycasting_scene_8h_source.html

### References

_No response_

### Additional information

_No response_