Make JoinSet::poll_join_next a public API

[bartlomieju]

Is your feature request related to a problem? Please describe.
We're migrating deno_core to use JoinSet instead of FuturesUnordered based on suggestions from @Darksonn. deno_core has a main poll_event_loop function that responsible for polling all spawned tasks. With FuturesUnordered polling them was as simple as:

while let Poll::Ready(...) = state.pending_ops.poll_next_unpin(cx) {
  ...
}

however with JoinSet more ceremony (and pinning) is required:

loop {
  let item = {
    let next = std::pin::pin!(context_state.pending_ops.join_next());
    let Poll::Ready(Some(item)) = next.poll(cx) else {
      break;
    };
    item
  };
  ...
}

Describe the solution you'd like
Make JoinSet::poll_join_next API public, which would change the example above to:

while Poll::Ready(Some(item)) = context_state.pending_ops.poll_join_next(cx) {
  ...
};

Describe alternatives you've considered
Keep current API as is without changes.

[Darksonn]

There's already a PR for this: #5721.

[bartlomieju]

Thanks, I'm gonna close the issue then.