[Firestore][POC] LoadBundle progress async stream

Added a proof-of-concept API for reporting bundle loading task progress via an AsyncThrowingStream.

Example Usage:

let bundleData: Data = // some Data()
do {
  for try await progress in Firestore.firestore().loadBundle(bundleData).taskProgress {
    print("LoadBundleTask Progress: \(progress)")
  }
  print("LoadBundleTask done.")
} catch {
  print("LoadBundleTask Error: \(error)")
}

Firestore/Swift/Source/AsyncAwait/LoadBundleTask+AsyncAwait.swift

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2022 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import Foundation
+import FirebaseFirestore
+
+@available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
+public extension LoadBundleTask {
+  var taskProgress: AsyncThrowingStream<LoadBundleTaskProgress, Error> {
+    AsyncThrowingStream(bufferingPolicy: .bufferingNewest(1)) { continuation in
+      let observerHandle: LoadBundleObserverHandle = self.addObserver { progress in
+        switch progress.state {
+        case .error:
+          guard let error = progress.error else {
+            fatalError("Unknown progress error.")
+          }
+          continuation.finish(throwing: error)
+        case .success:
+          continuation.yield(progress)
+          continuation.finish()
+        case .inProgress:
+          continuation.yield(progress)
+        @unknown default:
+          fatalError("Unknown progress state: \(progress.state)")
+        }
+      }
+      continuation.onTermination = { @Sendable _ in
+        self.removeObserverWith(handle: observerHandle)
+      }
+    }
+  }
+}