Support for stale-while-invalidate

RFC-5861 introduces a cache-control extension "stale-while-invalidate":

When present in an HTTP response, the stale-while-revalidate Cache-
Control extension indicates that caches MAY serve the response in
which it appears after it becomes stale, up to the indicated number
of seconds.

If a cached response is served stale due to the presence of this
extension, the cache SHOULD attempt to revalidate it while still
serving stale responses (i.e., without blocking).

We're talking to a service that uses this directive, which is causing our client to log an error:

cachecontrol.controller | Ignoring unknown cache-control directive: stale-while-revalidate

AFAICT stale-while-invalidate is not an official directive and is not part of RFC-7234 (caching), but there might be value in supporting it in cachecontrol nonetheless.

Start with the cachecontrol.controller handling mentioned in the error and compare the RFC-5861 and RFC-7234 references, noting the issue's inconsistent use of “invalidate” and “revalidate.” Done should define and test the intended handling of this directive so valid input is no longer incorrectly reported as unknown.