Support DNS stamps

[jedisct1]

DNS stamps encode all the required information to connect to a secure DNS resolver as a single string.

A DoH stamp includes SPKI hashes and bootstrap IP addresses in addition to the URL.

Stamps prevent users from ignoring SPKI, bootstrap with the correct servers, require only a single string to copy/paste, and can be easily listed in documentations and text files: https://github.com/DNSCrypt/dnscrypt-resolvers/blob/master/v2/public-resolvers.md

It would be nice if Intra supported DNS stamps.