Added derive Reflect to UntypedHandle and UntypedAssetId (bevyengine#18827)

# Objective

- We have the ability to serialize/deserialize `Handle<T>` with
[`TypedReflectSerializer`](https://docs.rs/bevy/latest/bevy/reflect/serde/struct.TypedReflectSerializer.html)
and
[`TypedReflectDeserializer`](https://docs.rs/bevy/latest/bevy/reflect/serde/struct.TypedReflectDeserializer.html),
but it is not possible for `UntypedHandle`.
- `Handle<T>` already has the `Reflect` derive, so it sounds coherent to
also have this derive also on the untyped API

## Solution

- Add the `Reflect` derive macro to both `UntypedHandle` and `
UntypedAssetId`.

## Testing

- I used a custom processor to handle the serialization based on the
example of
[`TypedReflectSerializer`](https://docs.rs/bevy/latest/bevy/reflect/serde/struct.TypedReflectSerializer.html)
(see [source
code](https://docs.rs/bevy_reflect/0.15.3/src/bevy_reflect/serde/ser/serializer.rs.html#149))

Co-authored-by: Frédéric Vauchelles <frederic.vauchelles@outlook.com>

Author: 2 people

crates/bevy_asset/src/handle.rs

@@ -292,7 +292,7 @@ impl<A: Asset> From<&mut Handle<A>> for UntypedAssetId {
 /// to be stored together and compared.
 ///
 /// See [`Handle`] for more information.
-#[derive(Clone)]
+#[derive(Clone, Reflect)]
 pub enum UntypedHandle {
     /// A strong handle, which will keep the referenced [`Asset`] alive until all strong handles are dropped.
     Strong(Arc<StrongHandle>),

crates/bevy_asset/src/id.rs

@@ -164,7 +164,7 @@ impl<A: Asset> From<AssetIndex> for AssetId<A> {
 /// An "untyped" / "generic-less" [`Asset`] identifier that behaves much like [`AssetId`], but stores the [`Asset`] type
 /// information at runtime instead of compile-time. This increases the size of the type, but it enables storing asset ids
 /// across asset types together and enables comparisons between them.
-#[derive(Debug, Copy, Clone)]
+#[derive(Debug, Copy, Clone, Reflect)]
 pub enum UntypedAssetId {
     /// A small / efficient runtime identifier that can be used to efficiently look up an asset stored in [`Assets`]. This is
     /// the "default" identifier used for assets. The alternative(s) (ex: [`UntypedAssetId::Uuid`]) will only be used if assets are