Does Firebase Cloud Messaging Composer support Creating Messaging with Custom sound?

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

* Unity editor version: 2019.4.28f1
* Firebase Unity SDK version: 8.2.0
* Source you installed the SDK: Unity Package Manager
* Problematic Firebase Component: FCM
* Other Firebase Components in use: Analytics
* Additional SDKs you are using: None
* Platform you are using the Unity editor on: Mac
* Platform you are targeting: iOS
* Scripting Runtime: IL2CPP

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

Hi,

Can we send custom sound attribute as part of Firebase Notification payload using the Firebase Composer? Currently, the only way I see I can override the `sound` attribute is via the DATA payload (Custom data key/value pairs), however, this will force the app to be responsible for processing data messages and ONLY when is in the foreground.

I am looking for ways we can leverage the Firebase Console without going through an alternative server or poxy in order to support and play custom sound for remote notifications (i.e. app is in the background or not open at all)

Thanks in advanced
Dioni