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.

cmd/cli/nextdns.go

@@ -0,0 +1,31 @@
+package cli
+
+import (
+	"fmt"
+
+	"github.com/Control-D-Inc/ctrld"
+)
+
+const nextdnsURL = "https://dns.nextdns.io"
+
+func generateNextDNSConfig() {
+	if nextdns == "" {
+		return
+	}
+	mainLog.Load().Info().Msg("generating ctrld config for NextDNS resolver")
+	cfg = ctrld.Config{
+		Listener: map[string]*ctrld.ListenerConfig{
+			"0": {
+				IP:   "0.0.0.0",
+				Port: 53,
+			},
+		},
+		Upstream: map[string]*ctrld.UpstreamConfig{
+			"0": {
+				Type:     ctrld.ResolverTypeDOH3,
+				Endpoint: fmt.Sprintf("%s/%s", nextdnsURL, nextdns),
+				Timeout:  5000,
+			},
+		},
+	}
+}