`retry`: Make retry interface a bit more flexible

[DaTrader]

The Req.Steps.retry/1 docs state that the retry-after header is only read in case of a 429 status, while the combo of the retry and retry_delay function options does not permit deciding on whether to retry say, based on the status, and then returning the delay retrieved from the retry-after.

Can this be made a little bit more flexible? I for instance need the functionality reserved for the 429 status in :retry_delay but for the 503.

Thanks