add custom DNS Resolver configuration

config.go

@@ -28,6 +28,7 @@ type Config struct {
 	AutoNAT   AutoNATConfig
 	Pubsub    PubsubConfig
 	Peering   Peering
+	DNS       DNSConfig
 
 	Provider     Provider
 	Reprovider   Reprovider

dns.go

@@ -0,0 +1,7 @@
+package config
+
+// DNSConfig specifies DNS resolution rules using custom resolvers
+type DNSConfig struct {
+	// CustomResolvers is a map of FQDNs to URLs for custom DNS resolution.
+	Resolvers map[string]string
+}