Add `::eventuals::Notification::HasBeenNotified()` or similar

[CodingCanuck]

::eventuals::Notification (a newly introduced type) is inspired by absl::Notification.

Let's add a method to ::eventuals::Notification called HasBeenNotified() or similar, similar to the absl::Notification equivalent: https://github.com/abseil/abseil-cpp/blob/547802119dfa444540d8f2bb8a9f50bf64252a0f/absl/synchronization/notification.h#L78-L80

This would make ::eventuals::Notification useful for signifying events that happen that other code might want to query periodically, e.g. a control loop that might periodically want to check if some interesting event has happened (like server shutdown or loop cancellation).