Add App Identifier to Push Notification Payload

We have built a custom push gateway to support push notifications for both the official Rocket.Chat app and our white-labeled app. To improve app-specific handling, it would be useful to include the app identifier in the notification payload, allowing the gateway to distinguish which app the notification is intended for.

Currently, the app identifier is already included in the payload send to the APN endpoint because there it is used as topic, but it is left out for the GCM endpoint:
https://github.com/RocketChat/Rocket.Chat/blob/c5f67341a5604dfbbba45c653d52a14590050292/apps/meteor/app/push/server/push.ts#L340-L348

Since the field topic is likely intended for notification topics, we propose adding a separate field appName (or similar) to the payload in addition to the topic field.

This allows a push gateway to decide how (or if) to handle a push notification.

Start in apps/meteor/app/push/server/push.ts around lines 340-348 and compare the APN and GCM payload construction. Confirm how the app identifier is currently used for the APN topic, then add the proposed separate identifier field to the GCM payload; done means a push gateway can distinguish the official and white-labeled apps without changing the topic field.