Request to add `set_on_key` for open3d.visualization.O3DVisualizer in Python

### 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

`open3d.visualization.O3DVisualizer` is an amazing out-of-box class as it provides many built-in features, however, there's no option for developers to add custom keyboard key handling to it. It is, therefore, requested that `set_on_key` should be exposed. I believe it is simple and clutter free to include this feature as `open3d.visualization.O3DVisualizer` inherits from `open3d.visualization.gui.Window` that has SetOnKeyEvent, so adding something like following to `cpp/pybind/visualization/o3dvisualizer.cpp` under `pybind_o3dvisualizer_definitions` should work just fine:

```
.def("set_on_key", &O3DVisualizer::SetOnKeyEvent,
"Sets a callback that will be called when a key is pressed. "
"The callback will be given the key event and should return "
"True if the event was handled, or False if it was not.",
"callback"_a);
```

### References

_No response_

### Additional information

I previously only used `pip` package of Open3D, so please let me know if this solution is not ideal or against design principles of Open3D.