Add dnsforge.de as built-in DNS provider

[jsapede]

Hey,

Any chance of adding dnsforge.de to the built-in providers? Its a German DNS resolver — no logging, DoT/DoH support, and it has a few filtering levels.

Id mainly be interested in the Hard variant (stricter blocking, no whitelists). Heres the info:

• Name: HardDNSForge
• DoT: hard.dnsforge.de
• DoH: https://hard.dnsforge.de/dns-query
• IPv4: 49.12.222.213, 88.198.122.154
• IPv6: 2a01:4f8:c17:2c61::213, 2a01:4f8:c013:5ec0::154

Full list of variants here: https://dnsforge.de/#dnsdaten

Would be nice to just set DNS_UPSTREAM_RESOLVERS=harddnsforge and be done with it instead of forking. Thanks!