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/NostrEventInfoFromPushNotification.swift

@@ -0,0 +1,49 @@
+//
+//  NostrEventInfoFromPushNotification.swift
+//  DamusNotificationService
+//
+//  Created by Daniel D’Aquino on 2023-11-13.
+//
+
+import Foundation
+
+/// The representation of a JSON-encoded Nostr Event used by the push notification server
+/// Needs to match with https://gitlab.com/soapbox-pub/strfry-policies/-/raw/433459d8084d1f2d6500fdf916f22caa3b4d7be5/src/types.ts
+struct NostrEventInfoFromPushNotification: Codable {
+    let id: String          // Hex-encoded
+    let sig: String         // Hex-encoded
+    let kind: NostrKind
+    let tags: [[String]]
+    let pubkey: String      // Hex-encoded
+    let content: String
+    let created_at: Int
+    
+    static func from(dictionary: [AnyHashable: Any]) -> NostrEventInfoFromPushNotification? {
+        guard let id = dictionary["id"] as? String,
+              let sig = dictionary["sig"] as? String,
+              let kind_int = dictionary["kind"] as? UInt32,
+              let kind = NostrKind(rawValue: kind_int),
+              let tags = dictionary["tags"] as? [[String]],
+              let pubkey = dictionary["pubkey"] as? String,
+              let content = dictionary["content"] as? String,
+              let created_at = dictionary["created_at"] as? Int else {
+            return nil
+        }
+        return NostrEventInfoFromPushNotification(id: id, sig: sig, kind: kind, tags: tags, pubkey: pubkey, content: content, created_at: created_at)
+    }
+    
+    func reactionEmoji() -> String? {
+        guard self.kind == NostrKind.like else {
+            return nil
+        }
+
+        switch self.content {
+        case "", "+":
+            return "❤️"
+        case "-":
+            return "👎"
+        default:
+            return self.content
+        }
+    }
+}