Upgrade S3 ListObjects version, allow >1000 model files

[the-david-oy]

This PR upgrades the version of ListObjectsRequest and ListObjectsResult, as per AWS recommendations here to switch to V2. It also enables Triton to load all files in an S3 bucket, beyond the page limit of 1000.

[GuanLuo]

Put this before the success handling, so the failure handle is a bit more obvious:

if (not_success) {
  return error;
}
// Check content..

[the-david-oy]

Great idea! Done.

[kthui]

I think @GuanLuo means this?

while (!done_listing) {
  auto list_objects_outcome = client_->ListObjectsV2(objects_request);

  if (!list_objects_outcome.IsSuccess()) {
    return Status(
        Status::Code::INTERNAL,
        "Could not list contents of directory at " + true_path +
            " due to exception: " +
            list_objects_outcome.GetError().GetExceptionName() +
            ", error message: " +
            list_objects_outcome.GetError().GetMessage());
  }

  const auto& list_objects_result = list_objects_outcome.GetResult();
  for (const auto& s3_object : list_objects_result.GetContents()) {
    // In the case of empty directories, the directory itself will appear here
    if (s3_object.GetKey().c_str() == full_dir) {
      continue;
    }
  
    // We have to make sure that subdirectory contents do not appear here
    std::string name(s3_object.GetKey().c_str());
    int item_start = name.find(full_dir) + full_dir.size();
    // S3 response prepends parent directory name
    int item_end = name.find("/", item_start);
  
    // Let set take care of subdirectory contents
    std::string item = name.substr(item_start, item_end - item_start);
    contents->insert(item);
  
    // Fail-safe check to ensure the item name is not empty
    if (item.empty()) {
      return Status(
          Status::Code::INTERNAL,
          "Cannot handle item with empty name at " + true_path);
    }
  }
  // If there are more pages to retrieve, set the marker to the next page.
  if (list_objects_result.GetIsTruncated()) {
    objects_request.SetContinuationToken(
        list_objects_result.GetNextContinuationToken());
  } else {
    done_listing = true;
  }
}

[GuanLuo]

Yes, this have one less nesting for the success case

[the-david-oy]

Got it, that makes sense! Thanks for clarifying, @kthui and @GuanLuo. Done.

[the-david-oy]

Still passes CI after removing the nesting. Please review when you get a chance!