fix: set notification center delegate to plugin self*

Addresses generated notifications not showing on foreground app

Author: cybex-dev

ios/Classes/SwiftTwilioVoicePlugin.swift

@@ -75,6 +75,8 @@ public class SwiftTwilioVoicePlugin: NSObject, FlutterPlugin,  FlutterStreamHand
 
         voipRegistry.delegate = self
         voipRegistry.desiredPushTypes = Set([PKPushType.voIP])
+        
+        UNUserNotificationCenter.current().delegate = self
 
         let appDelegate = UIApplication.shared.delegate
         guard let controller = appDelegate?.window??.rootViewController as? FlutterViewController else {