429 and 503 codes can, and often do (e.g. all Google Cloud
Storage URLs can), return a Retry-After header with the error,
indicating how long to wait, asd either a date, or in seconds,
before retrying again. If it is not respected by, for example,
using our default backoff stratetgy instead, chances of success
are very unlikely.

Some references:
    * https://datatracker.ietf.org/doc/html/rfc6585
    * https://datatracker.ietf.org/doc/html/rfc7231#section-7.1.3

This adds an AVOption to respect that header.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>