Show onion redirects in the tor circuit panel

In #41600 (closed) we wanted to show the original domain for automated onion redirects, via "alt-svc" or "onion-location". See option 2 in #41600 (comment 2874624) and the figma mock-up.

However there were two issues:

1. I couldn't find a way to determine from the current <xul:browser> whether there was a redirect, and what the original domain name was. For either alt-svc or onion-location.
2. In the mockups, we show the original domain name as a link, so presumably we want the user to be able to open it without the redirect.

So if we want this feature we need to figure this out, and possibly add some backend code.