Add AsyncSeekForwardExt trait to allows a similar API to the previous Bevy version (bevyengine#16027)

# Objective

This PR introduces an `AsyncSeekForwardExt` trait, which I forgot in my
previous PR bevyengine#14194.

This new trait is analogous to `AsyncSeekExt` and allows all
implementors of `AsyncSeekForward` to directly use the `seek_forward`
function in async contexts.

## Solution

- Implement a new `AsyncSeekForwardExt` trait
- Automatically implement this trait for all types that implement
`AsyncSeekForward`

## Showcase

This new trait allows a similar API to the previous Bevy version:

```rust
#[derive(Default)]
struct UniverseLoader;

#[derive(Asset, TypePath, Debug)]
struct JustALilAsteroid([u8; 128]);

impl AssetLoader for UniverseLoader {
    type Asset = JustALilAsteroid;
    type Settings = ();
    type Error = std::io::Error;

    async fn load<'a>(
        &'a self,
        reader: &'a mut Reader<'a>,
        _settings: &'a Self::Settings,
        _context: &'a mut LoadContext<'_>,
    ) -> Result<Self::Asset, Self::Error> {
        // read the asteroids entry table
        let entry_offset: u64 = /* ... */;
        let current_offset: u64 = reader.seek_forward(0).await?;

        // jump to the entry
        reader.seek_forward(entry_offset - current_offset).await?;

        let mut asteroid_buf = [0; 128];
        reader.read_exact(&mut asteroid_buf).await?;

        Ok(JustALilAsteroid(asteroid_buf))
    }
    
    fn extensions(&self) -> &[&str] {
        &["celestial"]
    }
}
```

Author: ldubos

crates/bevy_asset/src/io/mod.rs

@@ -23,6 +23,7 @@ pub use source::*;
 
 use alloc::sync::Arc;
 use bevy_utils::{BoxedFuture, ConditionalSendFuture};
+use core::future::Future;
 use core::{
     mem::size_of,
     pin::Pin,
@@ -120,6 +121,40 @@ impl<T: ?Sized + AsyncSeekForward + Unpin> AsyncSeekForward for Box<T> {
     }
 }
 
+/// Extension trait for [`AsyncSeekForward`].
+pub trait AsyncSeekForwardExt: AsyncSeekForward {
+    /// Seek by the provided `offset` in the forwards direction, using the [`AsyncSeekForward`] trait.
+    fn seek_forward(&mut self, offset: u64) -> SeekForwardFuture<'_, Self>
+    where
+        Self: Unpin,
+    {
+        SeekForwardFuture {
+            seeker: self,
+            offset,
+        }
+    }
+}
+
+impl<R: AsyncSeekForward + ?Sized> AsyncSeekForwardExt for R {}
+
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct SeekForwardFuture<'a, S: Unpin + ?Sized> {
+    seeker: &'a mut S,
+    offset: u64,
+}
+
+impl<S: Unpin + ?Sized> Unpin for SeekForwardFuture<'_, S> {}
+
+impl<S: AsyncSeekForward + Unpin + ?Sized> Future for SeekForwardFuture<'_, S> {
+    type Output = futures_lite::io::Result<u64>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let offset = self.offset;
+        Pin::new(&mut *self.seeker).poll_seek_forward(cx, offset)
+    }
+}
+
 /// A type returned from [`AssetReader::read`], which is used to read the contents of a file
 /// (or virtual file) corresponding to an asset.
 ///