FCM - Notification Image not displaying

### Describe your environment

* Operating System version: macOS 13.0 (22A380)
* Firebase SDK version: 9.1.1
* Library version: 9.1.1
* Firebase Product: Cloud Messaging (auth, database, storage, etc)

#### Steps to reproduce:

Images are not displayed on devices (both iOS and Android) when sending notifications with FCM.
I set the image URL using the Notification Builder.

```
List fcmTokens = List.of(...);

Notification notification = Notification.builder().setTitle("Hello").setBody("world").setImage("https://e-cdns-images.dzcdn.net/images/cover/f2d3db07f3f045a048cdacf53b5cf9dd/250x250-000000-80-0-0.jpg").build();

MulticastMessage message = MulticastMessage.builder()
.setNotification(notification)
.addAllTokens(fcmTokens).build();

BatchResponse response = FirebaseMessaging.getInstance().sendMulticast(message);
```

Note that I receive the notification on the devices, only the image is not displayed. I am able to see the image in the notification when sending it using the Firebase Console.

When investigating the issue, I discovered that other SDKs use `imageUrl` instead of `image` attribute.
For example, for NodeJS: https://firebase.google.com/docs/reference/admin/node/firebase-admin.messaging.notification.md#notificationimageurl

The Java SDK still uses the `image` key instead of `imageUrl`: https://github.com/firebase/firebase-admin-java/blob/master/src/main/java/com/google/firebase/messaging/Notification.java#L33

参照されている src/main/java/com/google/firebase/messaging/Notification.java から始め、その image フィールドをリンク先の Node.js の imageUrl リファレンスと比較します。FCM リクエスト内で Notification がどのように表現されるかを追跡し、その後、提供されている multicast の例を iOS と Android で検証します。通知画像が表示されれば完了です。

索引モデルが issue の本文から書いたものです。