Add DNSCrypt server-side support

[ameshkov]

See the original feature request in the dnsproxy repo:
AdguardTeam/dnsproxy#44

It is already highly upvoted so I think it makes sense to assign it to a milestone already.

[ameshkov]

This would be quite problematic to add and I don't see that much desire for this to happen.

There's always DOH/DOT for those who would like encryption.

[ameshkov]

Okay, not that problematic after all since DNSCrypt support has been added to dnsproxy: AdguardTeam/dnsproxy@39abe80