[Feature] Does notification banner show up on iOS app running in foreground?

### [REQUIRED] Please fill in the following fields:

* Unity editor version: 2019.4.6f1
* Firebase Unity SDK version: 7.1.0
* Source you installed the SDK: .unitypackage
* Problematic Firebase Component: Cloud Messaging
* Other Firebase Components in use:
* Additional SDKs you are using:
* Platform you are using the Unity editor on: Mac
* Platform you are targeting: iOS
* Scripting Runtime: Mono

### [REQUIRED] Please describe the question here:

When I build quickstart-unity/messaging/testapp on iOS and send a message while the app is in foreground, I expected to see a notification banner to show up, but actually it didn't. Only MessageReceived event was fired.

Is this an intended behavior?
If so, is there any way to show the banner on an app in foreground?

Inizia da quickstart-unity/messaging/testapp e traccia il percorso con cui il caso iOS in foreground raggiunge l’evento MessageReceived. Conferma se l’assenza del banner di notifica è prevista per Firebase Cloud Messaging, quindi determina se l’esempio o la relativa documentazione richiedono una modifica per spiegare o supportare i banner in foreground.

Scritto dal modello di indicizzazione a partire dal testo della issue.