Introduce asynchronous event notification options

Currently, it's only possible to provide a custom executor when firing an event asynchronously - see also javax.enterprise.event.Event.fireAsync(U, Executor). It might be useful to provide other implementation-specific options, e.g. notification timeout.

Therefore, I suggest to introduce NotificationOptions interface:

public interface NotificationOptions {
    Executor getExecutor();
    // Implementation-specific options
    Object get(String optionName);
}

and change the method signature to:

<U extends T> CompletionStage<U>  fireAsync(U event, NotificationOptions options);

In the future, if any implementation-specific configuration proves to be useful and worth standardizing, we may simply add a new method to NotificationOptions, e.g.

Duration getTimeout();

instead of adding more and more params to Event.fireAsync().

We could also introduce some convenient static methods on NotificationOptions, e.g.:

void fireEvents(Event event, Executor executor) {
    event.fireAsync(new Foo(), NotificationOptions.ofExecutor(executor));
}