Bundler: Provide option to specifiy the executable or script to be called on startup

## Feature Request

I have a software which also needs a backend process to run before opening the GUI frontend. So far, I used a (powershell / bash) script for handling this. With the bundler, the final installation package (e.g. MSI) always generates a start shortcut calling the main GUI application. It would be nice to have an option to call a different command.

## Implement Suggestion

Provide a config entry in `Dioxus.toml`. Maybe something like:

```toml
[bundle]
startup_cmd = "startup.ps1"
````

Alternatively this could be added to the sections `[bundle.windows]`, `[bundle.macos]`, etc. to provide platform specific startup commands.