feat: add method to get the stream cancellation token (#80)

In some cases, the stream is transferred to a different context, making
it impossible to cancel from the current one. By making the
download_task_cancellation_token public, you can clone the token
beforehand, ensuring that cancellation remains possible regardless of
the context shift

---------

Co-authored-by: Austin Schey <[email protected]>

src/lib.rs

@@ -341,6 +341,11 @@ impl<P: StorageProvider> StreamDownload<P> {
         self.download_task_cancellation_token.cancel();
     }
 
+    /// Get the [`CancellationToken`] for the download task.
+    pub fn get_cancellation_token(&self) -> CancellationToken {
+        self.download_task_cancellation_token.clone()
+    }
+
     async fn from_make_stream<S, F, Fut>(
         make_stream: F,
         storage_provider: P,