Report reasons why sxg-rs might not sign

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()](https://github.com/google/sxg-rs/blob/d16e66a2b8d5e0c069ec70af198dd95f640321d8/sxg_rs/src/lib.rs#L94) 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.