refactor(indexer-alt): graceful shutdown returns values

## Description

Allow `graceful_shutdown` to accept tasks that return something other
than `()`.

## Test plan

CI

crates/sui-indexer-alt/src/task.rs

@@ -9,10 +9,12 @@ use tokio_util::sync::CancellationToken;
 
 /// Manages cleanly exiting the process, either because one of its constituent services has stopped
 /// or because an interrupt signal was sent to the process.
-pub async fn graceful_shutdown(
-    services: impl IntoIterator<Item = JoinHandle<()>>,
+///
+/// Returns the exit values from all services that exited successfully.
+pub async fn graceful_shutdown<T>(
+    services: impl IntoIterator<Item = JoinHandle<T>>,
     cancel: CancellationToken,
-) {
+) -> Vec<T> {
     // If the service is naturalling winding down, we don't need to wait for an interrupt signal.
     // This channel is used to short-circuit the await in that case.
     let (cancel_ctrl_c_tx, cancel_ctrl_c_rx) = oneshot::channel();
@@ -24,20 +26,23 @@ pub async fn graceful_shutdown(
             _ = signal::ctrl_c() => cancel.cancel(),
         }
 
-        Ok(())
+        None
     };
 
     tokio::pin!(interrupt);
     let futures: Vec<_> = services
         .into_iter()
-        .map(Either::Left)
+        .map(|s| Either::Left(Box::pin(async move { s.await.ok() })))
         .chain(iter::once(Either::Right(interrupt)))
         .collect();
 
     // Wait for the first service to finish, or for an interrupt signal.
-    let (_, _, rest) = future::select_all(futures).await;
+    let (first, _, rest) = future::select_all(futures).await;
     let _ = cancel_ctrl_c_tx.send(());
 
     // Wait for the remaining services to finish.
-    let _ = future::join_all(rest).await;
+    let mut results = vec![];
+    results.extend(first);
+    results.extend(future::join_all(rest).await.into_iter().flatten());
+    results
 }