Session should be usable as a `futures::Stream`

[dbolduc]

Most of our types like the Paginator, Poller, storage::ReadObjectResponse are compatible with futures::Stream. e.g.

google-cloud-rust/src/storage/src/read_object.rs

Line 113 in a314eee

pub fn into_stream(self) -> impl Stream<Item = Result<bytes::Bytes>> + Unpin {

We offer an unstable-stream feature on these crates, and a conversion function to change them into something that impl futures::Stream.

(This stuff is feature gated because the futures crate is not generally available (GA), as its version is 0.x.y. We cannot have unstable types in our stable public API surface.)

We should do the same thing for a pubsub::subscriber::Session.

---

As followup, we can simplify the examples using streaming primitives, like timing out a stream with StreamExt::take_until:

use futures::StreamExt;

let deadline = tokio::time::sleep_until(Duration::from_secs(10));
let stream = session.into_stream().take_until(deadline);
while let Some(m, h) = stream.next().transpose()? { ... }

https://docs.rs/futures/latest/futures/stream/trait.StreamExt.html#method.take_until