Use retryable HTTP client (for 50x) errors

To make clients like [gh](https://github.com/cli/cli) more robust, it would be great to have HTTP retries especially on 50x errors. A library like [go-retryablehttp](https://github.com/hashicorp/go-retryablehttp) could be used for that, but that would be clearly a breaking API change. 

Do you have thoughts on that? 