Need a way to call the `setup()` of the plugins that have been registered in the App manually

[jinleili]

What problem does this solve or what need does it fill?

Due to changes in #7046, it is now necessary to have a way to call the setup function of the plugins that have been registered in the App when using an external event loop to drive a bevy App.

bevy/crates/bevy_app/src/app.rs

Lines 295 to 300 in 1ffeff1

	// temporarily remove the plugin registry to run each plugin's setup function on app.
	let mut plugin_registry = std::mem::take(&mut app.plugin_registry);
	for plugin in &plugin_registry {
	plugin.setup(&mut app);
	}
	std::mem::swap(&mut app.plugin_registry, &mut plugin_registry);

What solution would you like?

Make plugin_registry public?

What alternative(s) have you considered?

Move relevant code above into a separate pub fn?

Additional context

External event loop demo: https://github.com/jinleili/bevy-in-app