souphttpclientsink: Add the retry and retry-delay properties

souphttpclientsink: Add the retry and retry-delay properties

These allow a failed request to be retried after the given number of seconds
instead of failing the pipeline. Take account of the Retry-After header if
present. Add retries parameter that controls the number of times an HTTP
request will be retried before failing.

https://bugzilla.gnome.org/show_bug.cgi?id=756318