HTTP retry on 503 status code

[dentiny]

I saw discussion thread: duckdb/duckdb#6153, which mentions 503 is a status code which indicates slow response and should be retried.

Copy from AWS official doc:

If you are not using an AWS SDK, you should implement retry logic when receiving the HTTP 503 error.

Ref: https://docs.aws.amazon.com/AmazonS3/latest/userguide/optimizing-performance-design-patterns.html#optimizing-performance-timeouts-retries

Other reference docs: https://docs.digitalocean.com/products/spaces/details/limits/#:~:text=Applications%20should%20retry%20with%20exponential%20backoff%20on%20503%20Slow%20Down%20errors.
(more in the discussion thread)

[dentiny]

The error message doesn't look related to my change.