Fix non-main thread callback issue (#13371)

Author: paulb777

FirebaseStorage/Sources/Internal/StorageInternalTask.swift

@@ -47,12 +47,18 @@ class StorageInternalTask: StorageTask {
       self.fetcher = fetcher
 
       Task {
+        let callbackQueue = reference.storage.fetcherService != nil ?
+          reference.storage.fetcherServiceForApp.callbackQueue : DispatchQueue.main
         do {
           let data = try await self.fetcher?.beginFetch()
-          completion?(data, nil)
+          callbackQueue?.async {
+            completion?(data, nil)
+          }
         } catch {
-          completion?(nil, StorageErrorCode.error(withServerError: error as NSError,
-                                                  ref: self.reference))
+          callbackQueue?.async {
+            completion?(nil, StorageErrorCode.error(withServerError: error as NSError,
+                                                    ref: self.reference))
+          }
         }
       }
     }

FirebaseStorage/Sources/Storage.swift

@@ -266,7 +266,7 @@ import FirebaseCore
 
   // MARK: - Internal and Private APIs
 
-  private var fetcherService: GTMSessionFetcherService?
+  var fetcherService: GTMSessionFetcherService?
 
   var fetcherServiceForApp: GTMSessionFetcherService {
     guard let value = fetcherService else {