Retry-Middleware: allow smart retry with suggestion from server

[ducttse]

Hi, is there a plan for supporting retry middleware with minimum retry wait duration.
For example, when the server return RetryDelay struct RetryInfo the client can use this information to delay retrying to the server.
I will be happy to provide an implementation if we can agree on the payload that will be return from the server side when it want the client to retry after a period.