[Request] Add support for `ImGui.InputText()` callbacks, like for example the autocomplete callback for when the `ImGuiInputTextFlags.CallbackCompletion` flag is set

`ImGui.InputText()` has support for callbacks when certain flags are set, at least in C++. It would be very useful to have support for those callbacks on the C# side as well, as it seems they are currently not implemented. Please correct me if I'm wrong on this.