Allow enabling DoH

From IRC:

18:37 <blaze> does qtwebengine support DoH atm? I know chromium does, even in linux, but not sure when/if it got integrated
14:54 <carewolf> blaze no we disabled it, though we may turn it back on. Need to check who runs these new DNS servers, and how we get them. We don't want to just use Google's like Chrome does
16:24 <The-Compiler> carewolf: that'd be cool, got some questions/feature requests about it too - even just being able to enable it optionally via QWebEngineSettings (and perhaps setting the resolver via a command line option, or QWebEngineSettings method) would be great
16:40 <carewolf> yeah, but it seems at the moment to mostly be controlled by command line arguments. It appeared during the 5.15 updates, so it made sense to disable it, but I will take a look after 94 rebase is done. Feel free to open a JIRA issue on it