Feature/dns client configuration (#563)

Added host configurators for Linux, Windows, and macOS.

The host configurator will update the peer system configuration
 directing DNS queries according to its capabilities.

Some Linux distributions don't support split (match) DNS or custom ports,
 and that will be reported to our management system in another PR

client/internal/engine.go

@@ -139,7 +139,6 @@ func NewEngine(
 		networkSerial:  0,
 		sshServerFunc:  nbssh.DefaultSSHServer,
 		statusRecorder: statusRecorder,
-		dnsServer:      dns.NewDefaultServer(ctx),
 	}
 }
 
@@ -261,6 +260,14 @@ func (e *Engine) Start() error {
 
 	e.routeManager = routemanager.NewManager(e.ctx, e.config.WgPrivateKey.PublicKey().String(), e.wgInterface, e.statusRecorder)
 
+	if e.dnsServer == nil {
+		dnsServer, err := dns.NewDefaultServer(e.ctx, e.wgInterface)
+		if err != nil {
+			return err
+		}
+		e.dnsServer = dnsServer
+	}
+
 	e.receiveSignalEvents()
 	e.receiveManagementEvents()