add live activity start event (#216)

Author: JulianKahnert

Sources/APNSCore/LiveActivity/APNSLiveActivityNotificationEvent.swift

@@ -17,6 +17,9 @@ public struct APNSLiveActivityNotificationEvent: Hashable, Sendable {
     @usableFromInline
     internal let rawValue: String
 
+    /// Specifies that live activity should be started
+    public static let start = Self(rawValue: "start")
+    
     /// Specifies that live activity should be updated
     public static let update = Self(rawValue: "update")