Fire an event when a notification is received

[bcutter]

For initial background/motivation and details on requirement please see https://community.home-assistant.io/t/basic-notifications-via-ha-companion-app-issue-not-received-if-devices-not-online/414757.
➡ In a nutshell: this allows to work around this:

Apple’s push notification service generally stores the latest 1 notification, with some variation for threads that I can’t recall offhand. You should assume notifications sent while offline are not guaranteed to be received.

Basically this feature request is about having the same new Android HA companion app ✔ feature of notification received confirmation (see home-assistant/android#2705) in the iOS HA Companion app ❌ too.