luci-app-https-dns-proxy: Add LibreDNS as a DoH provider

LibreDNS (libredns.gr) is a Doh service provided by LibreOps(libreops.cc), a group
of hackers around the world that are (re-)decentralizing the net.
Trying offering distributed, free (as in freedom) services to the world.
Always based on Free Open Source Software.

Signed-off-by: Konstantinos Stamou (Dinos) <github@f3rr3t.net>