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

+package config
+
+// DNSConfig specifies custom resolvers using DoH
+type DNSConfig struct {
+	// DefaultResolver, if present, is a URL for the default DoH resolver.
+	// If empty, DNS resolution will use the system resolver.
+	DefaultResolver string `json:",omitempty"`
+	// CustomResolvers is a map of domains to URLs for custom DoH resolution.
+	CustomResolvers map[string]string `json:",omitempty"`
+}