Enable configurable Authorization header in DoH

[Shamar]

Reasoning about #112 I realized that firefox let me set an Authorization header for my DoH resolver by setting network.trr.credentials config in about:config, while (as far as I can read) RethinkDNS does not.

According to the documentation network.trr.credentials can "set credentials that will be used in the HTTP requests to the DoH end-point. It is the right-hand side content, the value, sent in the Authorization: request header."

With personal DoH sinkholes (like the ones provided by doh.cgi) a Basic Authorization configured at .htaccess level would limit the server load from freeloaders.

Just like with #112, beyond a change to firestack we should also change the DoH (and maybe ODoH) configuration UI for resolvers' URI.