Add support for broadcast push notifications

[jchambers]

Sounds like there's some new stuff in iOS 18: https://developer.apple.com/videos/play/wwdc2024/10069/

I haven't yet watched the video, but will update this issue when I better understand the scope of the feature and the work required to support it.

[pgus2024]

@jchambers - Can we please prioritize this?
We are blocked since we need to support broadcast live activity notifications.
I think there are 2 changes required-

1. We add the apns-channel-id header
2. We make token to be not mandatory in case apns-channel-id is present