Process cross-origin links for subresource preloading

[twifkak]

#57 limited its solution of #13 to same-origin preloads for efficiency's sake:

• Traffic costs for same-origin subrequests are probably cheaper than cross-origin.
• Most cross-origin hrefs won't have corresponding SXGs, so the preload processing would likely fail (and thus be in vain), while same-origin hrefs would likely have corresponding SXGs because sxg-rs is usually run on the whole origin.

To address this problem, we could mitigate the cost, by caching a per-origin boolean saying whether it supports SXG. Say, 1h expiry. On cache expiry/miss, process the link as if it supports SXG and update the cache accordingly.