Add SubApp::take_extract() (#16862)

# Objective

Fixes #16850

## Solution

Add a new function `SubApp::take_extract()`, similar to
`Option::take()`, which allows stealing the currently installed extract
function of a sub-app, with the intent to replace it with a custom one
calling the original one via `set_extract()`.

This pattern enables registering a custom "world sync" function similar
to the existing one `entity_sync_system()`, to run custom world sync
logic with mutable access to both the main and render worlds.

## Testing

`cargo r -p ci` currently doesn't build locally, event after upgrading
rustc to latest and doing a `cargo update`.

crates/bevy_app/src/sub_app.rs

@@ -166,6 +166,35 @@ impl SubApp {
         self
     }
 
+    /// Take the function that will be called by [`extract`](Self::extract) out of the app, if any was set,
+    /// and replace it with `None`.
+    ///
+    /// If you use Bevy, `bevy_render` will set a default extract function used to extract data from
+    /// the main world into the render world as part of the Extract phase. In that case, you cannot replace
+    /// it with your own function. Instead, take the Bevy default function with this, and install your own
+    /// instead which calls the Bevy default.
+    ///
+    /// ```
+    /// # use bevy_app::SubApp;
+    /// # let mut app = SubApp::new();
+    /// let default_fn = app.take_extract();
+    /// app.set_extract(move |main, render| {
+    ///     // Do pre-extract custom logic
+    ///     // [...]
+    ///
+    ///     // Call Bevy's default, which executes the Extract phase
+    ///     if let Some(f) = default_fn.as_ref() {
+    ///         f(main, render);
+    ///     }
+    ///
+    ///     // Do post-extract custom logic
+    ///     // [...]
+    /// });
+    /// ```
+    pub fn take_extract(&mut self) -> Option<ExtractFn> {
+        self.extract.take()
+    }
+
     /// See [`App::insert_resource`].
     pub fn insert_resource<R: Resource>(&mut self, resource: R) -> &mut Self {
         self.world.insert_resource(resource);