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/NotificationFormatter.swift

@@ -0,0 +1,48 @@
+//
+//  NotificationFormatter.swift
+//  DamusNotificationService
+//
+//  Created by Daniel D’Aquino on 2023-11-13.
+//
+
+import Foundation
+import UserNotifications
+
+struct NotificationFormatter {
+    static var shared = NotificationFormatter()
+    
+    // TODO: These is a very generic notification formatter. Once we integrate NostrDB into the extension, we should reuse various functions present in `HomeModel.swift`
+    func formatMessage(event: NostrEventInfoFromPushNotification) -> UNNotificationContent? {
+        let content = UNMutableNotificationContent()
+        if let event_json_data = try? JSONEncoder().encode(event),  // Must be encoded, as the notification completion handler requires this object to conform to `NSSecureCoding`
+           let event_json_string = String(data: event_json_data, encoding: .utf8) {
+            content.userInfo = [
+                "nostr_event_info": event_json_string
+            ]
+        }
+        switch event.kind {
+            case .text:
+                content.title = NSLocalizedString("Someone posted a note", comment: "Title label for push notification where someone posted a note")
+                content.body = event.content
+                break
+            case .dm:
+                content.title = NSLocalizedString("New message", comment: "Title label for push notifications where a direct message was sent to the user")
+                content.body = NSLocalizedString("(Contents are encrypted)", comment: "Label on push notification indicating that the contents of the message are encrypted")
+                break
+            case .like:
+                guard let reactionEmoji = event.reactionEmoji() else {
+                    content.title = NSLocalizedString("Someone reacted to your note", comment: "Generic title label for push notifications where someone reacted to the user's post")
+                    break
+                }
+                content.title = NSLocalizedString("New note reaction", comment: "Title label for push notifications where someone reacted to the user's post with a specific emoji")
+                content.body = String(format: NSLocalizedString("Someone reacted to your note with %@", comment: "Body label for push notifications where someone reacted to the user's post with a specific emoji"), reactionEmoji)
+                break
+            case .zap:
+                content.title = NSLocalizedString("Someone zapped you ⚡️", comment: "Title label for a push notification where someone zapped the user")
+                break
+            default:
+                return nil
+        }
+        return content
+    }
+}