Support s-maxage directive

[ekrekeler]

I noticed some trouble using httpx-caching where it wouldn't serve a cached result when the time in the "Expires" header had passed, even though s-maxage was set in Cache-Control header. CDNs use s-maxage and this should take precedence over the "max-age" directive which takes precedence over "Expires" header.

Refer to RFC 9111 Section 5.2.2.10.