cmd/cli: add nextdns mode

Adding --nextdns flag to "ctrld start" command for generating ctrld
config with nextdns resolver id, then use nextdns as an upstream.

config.go

@@ -323,7 +323,7 @@ func (uc *UpstreamConfig) UpstreamSendClientInfo() bool {
 	}
 	switch uc.Type {
 	case ResolverTypeDOH, ResolverTypeDOH3:
-		if uc.isControlD() {
+		if uc.isControlD() || uc.isNextDNS() {
 			return true
 		}
 	}
@@ -520,6 +520,16 @@ func (uc *UpstreamConfig) isControlD() bool {
 	return false
 }
 
+func (uc *UpstreamConfig) isNextDNS() bool {
+	domain := uc.Domain
+	if domain == "" {
+		if u, err := url.Parse(uc.Endpoint); err == nil {
+			domain = u.Hostname()
+		}
+	}
+	return domain == "dns.nextdns.io"
+}
+
 func (uc *UpstreamConfig) dohTransport(dnsType uint16) http.RoundTripper {
 	uc.transportOnce.Do(func() {
 		uc.SetupTransport()