feat(apple): Apply MDM changes to Configuration (#9169)

When the MDM installs a configuration payload to
`dev.firezone.firezone.network-extension`, the tunnel service will now
be notified of a change to its `managedDict`, applying the configuration
and updating `packetTunnelProvider`'s local copy so that it'll be
returned on the next configuration fetch from the UI.

Related: #4505

swift/apple/FirezoneNetworkExtension/ConfigurationManager.swift

@@ -21,16 +21,26 @@ class ConfigurationManager {
   // can cause deadlocks in rare cases.
   private var userDict: [String: Any?]
 
-  private var managedDict: [String: Any?] {
-    userDefaults.dictionary(forKey: managedDictKey) ?? [:]
-  }
+  private var managedDict: [String: Any?]
 
   private init() {
     userDefaults = UserDefaults.standard
     userDict = userDefaults.dictionary(forKey: userDictKey) ?? [:]
+    managedDict = userDefaults.dictionary(forKey: managedDictKey) ?? [:]
 
     migrateFirezoneId()
     Telemetry.firezoneId = userDict[Configuration.Keys.firezoneId] as? String
+
+    NotificationCenter.default.addObserver(
+      self,
+      selector: #selector(handleUserDefaultsChanged),
+      name: UserDefaults.didChangeNotification,
+      object: userDefaults
+    )
+  }
+
+  deinit {
+    NotificationCenter.default.removeObserver(self, name: UserDefaults.didChangeNotification, object: userDefaults)
   }
 
   // Save user-settable configuration
@@ -69,6 +79,10 @@ class ConfigurationManager {
     setFirezoneId(UUID().uuidString)
   }
 
+  @objc private func handleUserDefaultsChanged(_ notification: Notification) {
+    self.managedDict = userDefaults.dictionary(forKey: managedDictKey) ?? [:]
+  }
+
   private func saveUserDict() {
     userDefaults.set(userDict, forKey: userDictKey)
   }

swift/apple/FirezoneNetworkExtension/PacketTunnelProvider.swift

@@ -176,8 +176,8 @@ class PacketTunnelProvider: NEPacketTunnelProvider {
         completionHandler?(configurationPayload)
 
       case .setConfiguration(let configuration):
-        self.configuration = configuration
         ConfigurationManager.shared.setConfiguration(configuration)
+        self.configuration = ConfigurationManager.shared.toConfiguration()
         completionHandler?(nil)
 
       case .signOut: