Add a "start" event for Live Activities

[jchambers]

Evidently, the process for starting Live Activities has changed in the latest iOS release. The old documentation (https://developer.apple.com/documentation/activitykit/update-and-end-your-live-activity-with-remote-push-notifications) appears to have been replaced with https://developer.apple.com/documentation/activitykit/starting-and-updating-live-activities-with-activitykit-push-notifications, which now calls for explicit "start" events and makes no mention of "end" events.

It's unclear to me if this change is a replacement for the old behavior, an optional replacement, or a mandatory replacement. In any case, I think the safest thing to do is simply to add the new "start" event.

This fixes #1047.

[pushly-cj]

Just confirming that the "start" event is a new event introduced in iOS 17.2 that allows a Live Activity to be started via an APNS Live Activity Notification whereas before you could only start the activity via your app and perform "update" events via APNS. I.e., The "update" event is still relevant and separate from "start".

We are seeing that the "end" event is still supported via APNS regardless of it not being in the documentation.

[jchambers]

@pushly-cj Thank you—this is very helpful!