async `future` support for every entity waiting on events

## Brief feature description

A user may want to receive a sample from a subscriber packed into a future, so that they can consume the sample whenever it is ready. The usage could look like:

```rust
let future_sample = subscriber.async_receive();
// do stuff here
println!("received {:?}", sample.await);
```

Other constructs that are maybe interesting are:
* service discovery - wait until a thing is there or gone
* events - wait for an incoming event
* publisher & listeners - wait for participants

No files, tests, or entry points are named. Start by surveying how subscribers, service discovery, events, publishers, and listeners currently wait for activity, then define which entities and future-based APIs are in scope. Done should include an agreed scope and working async support for the selected cases.

Written by the indexing model from the issue text.