Make SavedAsset::get_labeled accept &str as label (bevyengine#11612)

# Objective

- SavedAsset's iter_labels returns ```&str```, however accessing
LabeledAssets requires ```CowArc<'static, str>```
- Although SavedAsset holds UntypedHandles in its hashmap of
LabeledAssets, they are inaccessible as LabeledAssets are casted to
SavedAsset or ErasedLoadedAsset, which don't contain their
UntypedHandles
- Adresses bevyengine#11609

## Solution

- Used Trait bounds to allow for either ```CowArc<'static, str>``` or
```&str``` to be used as a label in get_labeled and get_erased_labeled.
- Added method get_untyped_handle to get UntypedHandle from the
LabeledAsset.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_asset/src/saver.rs

@@ -1,8 +1,8 @@
 use crate::{io::Writer, meta::Settings, Asset, ErasedLoadedAsset};
-use crate::{AssetLoader, LabeledAsset};
+use crate::{AssetLoader, LabeledAsset, UntypedHandle};
 use bevy_utils::{BoxedFuture, CowArc, HashMap};
 use serde::{Deserialize, Serialize};
-use std::ops::Deref;
+use std::{borrow::Borrow, hash::Hash, ops::Deref};
 
 /// Saves an [`Asset`] of a given [`AssetSaver::Asset`] type. [`AssetSaver::OutputLoader`] will then be used to load the saved asset
 /// in the final deployed application. The saver should produce asset bytes in a format that [`AssetSaver::OutputLoader`] can read.
@@ -95,11 +95,12 @@ impl<'a, A: Asset> SavedAsset<'a, A> {
     }
 
     /// Returns the labeled asset, if it exists and matches this type.
-    pub fn get_labeled<B: Asset>(
-        &self,
-        label: impl Into<CowArc<'static, str>>,
-    ) -> Option<SavedAsset<B>> {
-        let labeled = self.labeled_assets.get(&label.into())?;
+    pub fn get_labeled<B: Asset, Q>(&self, label: &Q) -> Option<SavedAsset<B>>
+    where
+        CowArc<'static, str>: Borrow<Q>,
+        Q: ?Sized + Hash + Eq,
+    {
+        let labeled = self.labeled_assets.get(label)?;
         let value = labeled.asset.value.downcast_ref::<B>()?;
         Some(SavedAsset {
             value,
@@ -108,14 +109,25 @@ impl<'a, A: Asset> SavedAsset<'a, A> {
     }
 
     /// Returns the type-erased labeled asset, if it exists and matches this type.
-    pub fn get_erased_labeled(
-        &self,
-        label: impl Into<CowArc<'static, str>>,
-    ) -> Option<&ErasedLoadedAsset> {
-        let labeled = self.labeled_assets.get(&label.into())?;
+    pub fn get_erased_labeled<Q>(&self, label: &Q) -> Option<&ErasedLoadedAsset>
+    where
+        CowArc<'static, str>: Borrow<Q>,
+        Q: ?Sized + Hash + Eq,
+    {
+        let labeled = self.labeled_assets.get(label)?;
         Some(&labeled.asset)
     }
 
+    /// Returns the [`UntypedHandle`] of the labeled asset with the provided 'label', if it exists.
+    pub fn get_untyped_handle<Q>(&self, label: &Q) -> Option<&UntypedHandle>
+    where
+        CowArc<'static, str>: Borrow<Q>,
+        Q: ?Sized + Hash + Eq,
+    {
+        let labeled = self.labeled_assets.get(label)?;
+        Some(&labeled.handle)
+    }
+
     /// Iterate over all labels for "labeled assets" in the loaded asset
     pub fn iter_labels(&self) -> impl Iterator<Item = &str> {
         self.labeled_assets.keys().map(|s| &**s)