Add a buffered AsyncIterator helper struct

# Purpose

In most cases, receivers of event data just want a "stream of data" that comes from the event. We therefore want to add an API that enables a subscription to be turned into a Stream / AsyncIterator so that a task can be created which consumes data asynchronously.

# Outline

```rust
pub struct StreamingSubscription {}

impl std::async_iter::AsyncIterator for StreamingSubscription {
...
}

impl std::async_iter::IntoAsyncIterator for for S where S: Subscription {
type Item = T;
type IntoAsyncIter = StreamingSubscription;
...
}
```

Maybe we also need to say something about a buffer size or which buffer is to be used internally, maybe as an option for power users who want / need to control this.