Add experimental push notification support

I added support for the experimental push notifications feature. There are many improvements to be made, so this feature is currently opt-in only. If the user does not opt-in, their device tokens will not be sent out and thus they will receive no push notifications.

We should perform more testing on real-life staging environments before fully releasing this feature.

Testing
-------

Testing was done gradually during development.

Device: iOS simulators
iOS: 17
Damus version: A few different but recent prototypes
Rough coverage:
1. Checked that no device tokens are sent out when setting is off
2. Checked that I can successfully receive device tokens when feature is ON and set to localhost.
3. Checked sending test push notifications of types "note" (kind: 1), reaction (kind: 7) and DMs (kind 4) works and shows a generic but reasonable push notification message
4. Checked that clicking on the notifications above take the user to the correct screen

Closes: https://github.com/damus-io/damus/issues/67
Changelog-Added: Add experimental push notification support
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Signed-off-by: William Casarin <jb55@jb55.com>

DamusNotificationService/NotificationService.swift

@@ -0,0 +1,39 @@
+//
+//  NotificationService.swift
+//  DamusNotificationService
+//
+//  Created by Daniel D’Aquino on 2023-11-10.
+//
+
+import UserNotifications
+import Foundation
+
+class NotificationService: UNNotificationServiceExtension {
+
+    var contentHandler: ((UNNotificationContent) -> Void)?
+    var bestAttemptContent: UNMutableNotificationContent?
+
+    override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
+        self.contentHandler = contentHandler
+        
+        // Modify the notification content here...
+        guard let nostrEventInfoDictionary = request.content.userInfo["nostr_event"] as? [AnyHashable: Any],
+              let nostrEventInfo = NostrEventInfoFromPushNotification.from(dictionary: nostrEventInfoDictionary) else {
+            contentHandler(request.content)
+            return;
+        }
+        
+        if let improvedContent = NotificationFormatter.shared.formatMessage(event: nostrEventInfo) {
+            contentHandler(improvedContent)
+        }
+    }
+    
+    override func serviceExtensionTimeWillExpire() {
+        // Called just before the extension will be terminated by the system.
+        // Use this as an opportunity to deliver your "best attempt" at modified content, otherwise the original push payload will be used.
+        if let contentHandler = contentHandler, let bestAttemptContent =  bestAttemptContent {
+            contentHandler(bestAttemptContent)
+        }
+    }
+
+}