Make JoinSet::poll_join_next public

My use-case can be seen here apache/datafusion#6449.

I want to avoid needless Box::pin calls when I can just poll the underlying task.

tokio/src/task/join_set.rs

@@ -362,7 +362,7 @@ impl<T: 'static> JoinSet<T> {
     /// This can happen if the [coop budget] is reached.
     ///
     /// [coop budget]: crate::task#cooperative-scheduling
-    fn poll_join_next(&mut self, cx: &mut Context<'_>) -> Poll<Option<Result<T, JoinError>>> {
+    pub fn poll_join_next(&mut self, cx: &mut Context<'_>) -> Poll<Option<Result<T, JoinError>>> {
         // The call to `pop_notified` moves the entry to the `idle` list. It is moved back to
         // the `notified` list if the waker is notified in the `poll` call below.
         let mut entry = match self.inner.pop_notified(cx.waker()) {