docs: @BetaApi
    public final Operation addSignedUrlKeyBackendBucket(
        String backendBucket, SignedUrlKey signedUrlKeyResource) {
  
      AddSignedUrlKeyBackendBucketHttpRequest request =
          AddSignedUrlKeyBackendBucketHttpRequest.newBuilder()
              .setBackendBucket(backendBucket)
              .setSignedUrlKeyResource(signedUrlKeyResource)
              .build();
      return addSignedUrlKeyBackendBucket(request);
    }