This diff introduces changes to the Rust notification library to add an optional APNs push message payload.
To wake up the iOS app from the background we should send a message with the payload. In this case, a message will be with the `content-available` attribute, so we can do background information processing in the app.
Apple's [[ https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/pushing_background_updates_to_your_app | related documentation ]], apple [[ https://developer.apple.com/forums/thread/31296 | forum thread ]].
The message payload is optional in the library.
Related Linear task: [[ https://linear.app/comm/issue/ENG-1743/testing-apple-notifications-apns-library-in-tunnelbroker | ENG-1743 ]]