[th/dot-sni] support setting server_name (SNI) for DoT (DNS over TLS) with systemd-resolved
implements #528 (closed)
edit 20221017: btw, this only extends ipv[46].dns
property to accept a server name. It won't do anything for DNS servers obtained via DHCP, etc. Unclear how to handle that (if at all). Maybe a new property dns-over-tls-sni-fallback
could be added and used.