refactor(apple): Annotate update resources callback @MainActor (#8069)

Followup to the discussion on
https://github.com/firezone/firezone/pull/8064. By annotating the
callback that updates our Resources `@MainActor`, the compiler will
correctly warn us when we call it from a non-isolated context.

swift/apple/FirezoneKit/Sources/FirezoneKit/Managers/VPNConfigurationManager.swift

@@ -320,7 +320,7 @@ public class VPNConfigurationManager {
     updateInternetResourceState()
   }
 
-  func fetchResources(callback: @escaping (ResourceList) -> Void) {
+  func fetchResources(callback: @escaping @MainActor (ResourceList) -> Void) {
     guard session()?.status == .connected else { return }
 
     do {
@@ -338,7 +338,7 @@ public class VPNConfigurationManager {
           self.resourcesListCache = ResourceList.loaded(decoded)
         }
 
-        callback(self.resourcesListCache)
+        Task { await MainActor.run { callback(self.resourcesListCache) } }
       }
     } catch {
       Log.error(error)

swift/apple/FirezoneKit/Sources/FirezoneKit/Stores/Store.swift

@@ -160,7 +160,7 @@ public final class Store: ObservableObject {
 
   // Network Extensions don't have a 2-way binding up to the GUI process,
   // so we need to periodically ask the tunnel process for them.
-  func beginUpdatingResources(callback: @escaping (ResourceList) -> Void) {
+  func beginUpdatingResources(callback: @escaping @MainActor (ResourceList) -> Void) {
     Log.log("\(#function)")
 
     if self.resourcesTimer != nil {

swift/apple/FirezoneKit/Sources/FirezoneKit/Views/SessionView.swift

@@ -63,7 +63,6 @@ public final class SessionViewModel: ObservableObject {
   public func isInternetResourceEnabled() -> Bool {
     store.internetResourceEnabled()
   }
-
 }
 
 #if os(iOS)