Feature request: automatically update the quasi official upstream sources.

Hi,

Considering there are some quasi official upstream sources for DoH, DoT, and DNSCrypt:

https://dnscrypt.info/public-servers/
https://github.com/curl/curl/wiki/DNS-over-HTTPS#publicly-available-servers
https://dnsprivacy.org/wiki/display/DP/DNS+Privacy+Test+Servers

So, it should be better for dnsproxy can automatically or let user assign them in config-file/cmd-arg so that dnsproxy can obtain them.

Regards