code: public URL generatePresignedUrl(String bucketName, String key, int expirationInSeconds) {
          return this.generatePresignedUrl(bucketName, key, expirationInSeconds, HttpMethodName.GET);
      }
docs: Returns a pre-signed URL for accessing a Bos resource.
  
  @param bucketName The name of the bucket containing the desired object.
  @param key The key in the specified bucket under which the desired object is stored.
  @param expirationInSeconds The expiration after which the returned pre-signed URL will expire.
  @return A pre-signed URL which expires at the specified time, and can be
  used to allow anyone to download the specified object from Bos,
  without exposing the owner's Bce secret access key.