Support system.in_schedule() and system.on_startup() (#7790)

# Objective

Support the following syntax for adding systems:

```rust
App::new()
    .add_system(setup.on_startup())
    .add_systems((
        show_menu.in_schedule(OnEnter(GameState::Paused)),
        menu_ssytem.in_set(OnUpdate(GameState::Paused)),
        hide_menu.in_schedule(OnExit(GameState::Paused)),
    ))
```

## Solution

Add the traits `IntoSystemAppConfig{s}`, which provide the extension methods necessary for configuring which schedule a system belongs to. These extension methods return `IntoSystemAppConfig{s}`, which `App::add_system{s}` uses to choose which schedule to add systems to.

---

## Changelog

+ Added the extension methods `in_schedule(label)` and  `on_startup()` for configuring the schedule a system belongs to.

## Future Work

* Replace all uses of `add_startup_system` in the engine.
* Deprecate this method

crates/bevy_app/src/app.rs

@@ -1,4 +1,7 @@
-use crate::{CoreSchedule, CoreSet, Plugin, PluginGroup, StartupSet};
+use crate::{
+    CoreSchedule, CoreSet, IntoSystemAppConfig, IntoSystemAppConfigs, Plugin, PluginGroup,
+    StartupSet, SystemAppConfig,
+};
 pub use bevy_derive::AppLabel;
 use bevy_ecs::{
     prelude::*,
@@ -378,10 +381,18 @@ impl App {
     /// #
     /// app.add_system(my_system);
     /// ```
-    pub fn add_system<M>(&mut self, system: impl IntoSystemConfig<M>) -> &mut Self {
+    pub fn add_system<M>(&mut self, system: impl IntoSystemAppConfig<M>) -> &mut Self {
         let mut schedules = self.world.resource_mut::<Schedules>();
 
-        if let Some(default_schedule) = schedules.get_mut(&*self.default_schedule_label) {
+        let SystemAppConfig { system, schedule } = system.into_app_config();
+
+        if let Some(schedule_label) = schedule {
+            if let Some(schedule) = schedules.get_mut(&*schedule_label) {
+                schedule.add_system(system);
+            } else {
+                panic!("Schedule {schedule_label:?} does not exist.")
+            }
+        } else if let Some(default_schedule) = schedules.get_mut(&*self.default_schedule_label) {
             default_schedule.add_system(system);
         } else {
             let schedule_label = &self.default_schedule_label;
@@ -406,48 +417,28 @@ impl App {
     /// #
     /// app.add_systems((system_a, system_b, system_c));
     /// ```
-    pub fn add_systems<M>(&mut self, systems: impl IntoSystemConfigs<M>) -> &mut Self {
-        let mut schedules = self.world.resource_mut::<Schedules>();
-
-        if let Some(default_schedule) = schedules.get_mut(&*self.default_schedule_label) {
-            default_schedule.add_systems(systems);
-        } else {
-            let schedule_label = &self.default_schedule_label;
-            panic!("Default schedule {schedule_label:?} does not exist.")
-        }
-
-        self
-    }
-
-    /// Adds a system to the provided [`Schedule`].
-    pub fn add_system_to_schedule<M>(
-        &mut self,
-        schedule_label: impl ScheduleLabel,
-        system: impl IntoSystemConfig<M>,
-    ) -> &mut Self {
-        let mut schedules = self.world.resource_mut::<Schedules>();
-
-        if let Some(schedule) = schedules.get_mut(&schedule_label) {
-            schedule.add_system(system);
-        } else {
-            panic!("Provided schedule {schedule_label:?} does not exist.")
-        }
-
-        self
-    }
-
-    /// Adds a collection of system to the provided [`Schedule`].
-    pub fn add_systems_to_schedule<M>(
-        &mut self,
-        schedule_label: impl ScheduleLabel,
-        systems: impl IntoSystemConfigs<M>,
-    ) -> &mut Self {
+    pub fn add_systems<M>(&mut self, systems: impl IntoSystemAppConfigs<M>) -> &mut Self {
         let mut schedules = self.world.resource_mut::<Schedules>();
 
-        if let Some(schedule) = schedules.get_mut(&schedule_label) {
-            schedule.add_systems(systems);
-        } else {
-            panic!("Provided schedule {schedule_label:?} does not exist.")
+        match systems.into_app_configs().0 {
+            crate::InnerConfigs::Blanket { systems, schedule } => {
+                let schedule = if let Some(label) = schedule {
+                    schedules
+                        .get_mut(&*label)
+                        .unwrap_or_else(|| panic!("Schedule '{label:?}' does not exist."))
+                } else {
+                    let label = &*self.default_schedule_label;
+                    schedules
+                        .get_mut(label)
+                        .unwrap_or_else(|| panic!("Default schedule '{label:?}' does not exist."))
+                };
+                schedule.add_systems(systems);
+            }
+            crate::InnerConfigs::Granular(systems) => {
+                for system in systems {
+                    self.add_system(system);
+                }
+            }
         }
 
         self
@@ -472,7 +463,7 @@ impl App {
     ///     .add_startup_system(my_startup_system);
     /// ```
     pub fn add_startup_system<M>(&mut self, system: impl IntoSystemConfig<M>) -> &mut Self {
-        self.add_system_to_schedule(CoreSchedule::Startup, system)
+        self.add_system(system.in_schedule(CoreSchedule::Startup))
     }
 
     /// Adds a collection of systems to [`CoreSchedule::Startup`].
@@ -497,7 +488,7 @@ impl App {
     /// );
     /// ```
     pub fn add_startup_systems<M>(&mut self, systems: impl IntoSystemConfigs<M>) -> &mut Self {
-        self.add_systems_to_schedule(CoreSchedule::Startup, systems)
+        self.add_systems(systems.into_configs().in_schedule(CoreSchedule::Startup))
     }
 
     /// Configures a system set in the default schedule, adding the set if it does not exist.