feat(artifact): enable gcs ListObjects (#6409)

Signed-off-by: Tianchu Zhao <evantczhao@gmail.com>

workflow/artifacts/gcs/gcs.go

@@ -302,5 +302,21 @@ func uploadObject(client *storage.Client, bucket, key, localPath string) error {
 }
 
 func (g *ArtifactDriver) ListObjects(artifact *wfv1.Artifact) ([]string, error) {
-	return nil, fmt.Errorf("ListObjects is currently not supported for this artifact type, but it will be in a future version")
+	var files []string
+	err := waitutil.Backoff(defaultRetry,
+		func() (bool, error) {
+			log.Infof("GCS List bucekt: %s, key: %s", artifact.GCS.Bucket, artifact.GCS.Key)
+			client, err := g.newGCSClient()
+			if err != nil {
+				log.Warnf("Failed to create new GCS client: %v", err)
+				return isTransientGCSErr(err), err
+			}
+			defer client.Close()
+			files, err = listByPrefix(client, artifact.GCS.Bucket, artifact.GCS.Key, "")
+			if err != nil {
+				return isTransientGCSErr(err), err
+			}
+			return true, nil
+		})
+	return files, err
 }