feat(bevy_app): expose an API to perform updates for a specific sub-app.

[Soulghost]

Objective

• Fixes SubApp::extract cannot be called due to borrow checker #14003

Solution

• Expose an API to perform updates for a specific sub-app, so we can avoid mutable borrow the app twice.

Testing

• I have tested the API by modifying the code in the many_lights example with the following changes:

impl Plugin for LogVisibleLights {
    fn build(&self, app: &mut App) {
        let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
            return;
        };

        render_app.add_systems(Render, print_visible_light_count.in_set(RenderSet::Prepare));
    }

    fn finish(&self, app: &mut App) {
        app.update_sub_app_by_label(RenderApp);
    }
}

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Changelog

• add the update_sub_app_by_label API to App and SubApps.

[janhohenheim]

Added some suggestions for making the documentation more in line with the existing methods

[alice-i-cecile]

@anlumo does this resolve your concerns?

[alice-i-cecile]

Agreed on the doc nits, but this is a fundamentally reasonable method to exist.

[anlumo]

Looks good to me!