Feature request: Add mullvad DOH resolver

[Steve-Brule]

What's the feature 🧐

Please add the mullvad DOH resolver to the available options for unbound. Maybe add both the adblocking and regular resolvers as separate options? Mullvad provides both.

Extra information and references

https://mullvad.net/en/help/dns-over-https-and-dns-over-tls/

[leo15dev]

You can track #137 to know when the DoH had been implemented in Gluetun. Actually, I personally hope we can add the custom DoH or DoT in Gluetun.