cli: add tailscale dns query (#13368)

Updates tailscale/tailscale#13326

Adds a CLI subcommand to perform DNS queries using the internal DNS forwarder and observe its internals (namely, which upstream resolvers are being used).

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

net/dns/resolver/forwarder.go

@@ -834,6 +834,17 @@ func (f *forwarder) resolvers(domain dnsname.FQDN) []resolverAndDelay {
 	return cloudHostFallback // or nil if no fallback
 }
 
+// GetUpstreamResolvers returns the resolvers that would be used to resolve
+// the given FQDN.
+func (f *forwarder) GetUpstreamResolvers(name dnsname.FQDN) []*dnstype.Resolver {
+	resolvers := f.resolvers(name)
+	upstreamResolvers := make([]*dnstype.Resolver, 0, len(resolvers))
+	for _, r := range resolvers {
+		upstreamResolvers = append(upstreamResolvers, r.name)
+	}
+	return upstreamResolvers
+}
+
 // forwardQuery is information and state about a forwarded DNS query that's
 // being sent to 1 or more upstreams.
 //