[Enhancement] Get all delivered notifications from system tray

[thomasgalliker]

Summary

It‘s useful for some scenarios to get the list of delivered but not (yet) opened notifications which are currently displayed in the system tray.

This list may include push notifications as well as local notifications.

API Changes

    IEnumerable<Notification> deliveredNotifications = IFirebaseNotifications.GetDeliveredNotificationsAsync();

Intended Use Case

Provide a detailed example of where your proposal would be used and for what purpose.