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>

damus/Models/UserSettingsStore.swift

@@ -191,6 +191,12 @@ class UserSettingsStore: ObservableObject {
     
     @Setting(key: "always_show_onboarding_suggestions", default_value: false)
     var always_show_onboarding_suggestions: Bool
+
+    @Setting(key: "enable_experimental_push_notifications", default_value: false)
+    var enable_experimental_push_notifications: Bool
+    
+    @Setting(key: "send_device_token_to_localhost", default_value: false)
+    var send_device_token_to_localhost: Bool
     
     @Setting(key: "emoji_reactions", default_value: default_emoji_reactions)
     var emoji_reactions: [String]