Add a DNS server

A DNS server module carries DNS messages to one upstream; core builds the queries, reads the answers, picks servers and caches. The examples are module/dns/udp (datagrams) and module/dns/tcp and module/dns/dot (streams, through shared/dnsstream).

Kind	Builds from
spec.StreamDNSServerModule	NewStreamDNSServer(dialer spec.StreamDialer)
spec.DatagramDNSServerModule	NewDatagramDNSServer(dialer spec.DatagramDialer)

Both return a spec.DNSServer:

• Query(ctx, queryMessage) sends one wire-format message and returns the one answer. Core calls it from many goroutines at once.
• Close() releases the streams or sockets the server holds, never the dialer.

Rules

• Never dial by yourself. The dialer reaches the server through the outbound the config names; the module never sees the outbound.
• TLS is core's. A stream module that needs TLS returns true from NeedsTLS, and core folds the tls layer into the dialer.
• Reuse connections as the protocol allows; only the module knows how.

Register it with moduleRegistry.DNSServers().RegisterStream(<name>.NewStreamDNSServerModule) or RegisterDatagram.