Mark the fallbacks that have extra-info descriptors, so stem can use them
In #19129 (moved), atagar says:
For what it's worth stem's descriptor downloader doesn't use fallback directories because they lack extrainfo descriptors...
This is fine. Tor doesn't need it (nor does it need the DirPort). However, if you want the fallbacks to relieve DirAuth load from other sources such as stem then these are both issues.
We could easily add a 'extrainfo=1' to each fallback that supported extrainfo at the time we chose them. Then teach tor to parse or ignore it, because at the time tor is fetching extra-infos, it has already bootstrapped and has an up-to-date list.