Report reasons why sxg-rs might not sign

[twifkak]

If a SXG is not returned from the origin, try to diagnose why sxg-rs might have chosen not to sign. (Not everybody is using sxg-rs, but I think most are.)

Go through sxg-rs create_signed_exchange() and capture the reasons signing might not happen (e.g. Cache-Control: no-store). The extension should look for those reasons in the origin response and display a warning if present.