Add support for DoH Forwarder DNS servers

[JJF-1]

Hi, first of all, thank you for this great project!
It would be really helpful if SDNS could support DoH URLs as upstream forwarders in the config, similar to the existing DoT forwarding. This will allow SDNS to utilize more upstream servers.
Thanks for considering!

[semihalev]

Hi. Very good idea. I will check what i can.

[semihalev]

I added this support. I will release in v1.7.0 soon.