Support pre-signed URL for External Storage

[xluffy]

When I use S3 external storage, I must pass AWS_ACCESS_KEY and AWS_SECRET_KEY in params, and send to Kraken API.

But how to know you don't steal my keys?

You can support pre-signed URLs instead. My system will generate a pre-signed URL in 10 minutes expire. So Kraken could upload to my S3 by specifying a path. It's secure more than the current solution.

Please consider this solution.