title: Unity google-play-core review issue
body: <p>I added the google play core package as instructed in the repo, and added the code according to the documentation <a href="https://developer.android.com/guide/playcore/in-app-review/unity" rel="nofollow noreferrer">here</a></p>
  <p>I receive the following error: <code>CS0246: The type or namespace name 'PlayAsyncOperation&lt;,&gt;' could not be found (are you missing a using directive or an assembly reference?)</code></p>
  <p>This error shows at <code>_reviewManager.RequestReviewFlow();</code> and <code>_reviewManager.LaunchReviewFlow(_playReviewInfo)</code></p>
  <p>Any idea what could be the issue?</p>
  <p>My full code:</p>
  <pre><code>using System.Collections;
  using System.Collections.Generic;
  using UnityEngine;
  using Google.Play.Review;
  
  public class ReviewCodes : MonoBehaviour
  {
      private ReviewManager _reviewManager;
      private PlayReviewInfo _playReviewInfo;
  
  
      // Start is called before the first frame update
      void Start()
      {
          StartCoroutine(RequestReviews());
      }
  
      // Update is called once per frame
      void Update()
      {
          
      }
  
      IEnumerator RequestReviews()
      {
          // Create instance of ReviewManager
          _reviewManager = new ReviewManager();
  
          var requestFlowOperation = _reviewManager.RequestReviewFlow();
          yield return requestFlowOperation;
          if (requestFlowOperation.Error != ReviewErrorCode.NoError)
          {
              // Log error. For example, using requestFlowOperation.Error.ToString().
              yield break;
          }
          _playReviewInfo = requestFlowOperation.GetResult();
  
          var launchFlowOperation = _reviewManager.LaunchReviewFlow(_playReviewInfo);
          yield return launchFlowOperation;
          _playReviewInfo = null; // Reset the object
          if (launchFlowOperation.Error != ReviewErrorCode.NoError)
          {
              // Log error. For example, using requestFlowOperation.Error.ToString().
              yield break;
          }
          // The flow has finished. The API does not indicate whether the user
          // reviewed or not, or even whether the review dialog was shown. Thus, no
          // matter the result, we continue our app flow.
      }
  }
  </code></pre>