Suggestion: Health-check example

Async could provide a health check as an example since it is simple enough to implement and useful in general. Constraints

* Ping the server periodically (can be a http endpoint).
* Use only one timer.
* The timer should not be cancelled.
* On expiration the timer should exit with success.

The interface could look like

```cpp
co_await health_check(connection, std::chrono::seconds{1});
```

Start by inspecting the repository's existing examples and coroutine timer usage, then identify where a health_check example would fit. Use the proposed health_check(connection, duration) interface and verify that it periodically pings an HTTP endpoint with one uncancelled timer, whose expiration exits successfully.

Written by the indexing model from the issue text.