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_test.go

@@ -202,6 +202,9 @@ func TestEngine_UpdateNetworkMap(t *testing.T) {
 	}, nbstatus.NewRecorder())
 	engine.wgInterface, err = iface.NewWGIFace("utun102", "100.64.0.1/24", iface.DefaultMTU)
 	engine.routeManager = routemanager.NewManager(ctx, key.PublicKey().String(), engine.wgInterface, engine.statusRecorder)
+	engine.dnsServer = &dns.MockServer{
+		UpdateDNSServerFunc: func(serial uint64, update nbdns.Config) error { return nil },
+	}
 
 	type testCase struct {
 		name       string
@@ -551,6 +554,7 @@ func TestEngine_UpdateNetworkMapWithRoutes(t *testing.T) {
 			}
 
 			engine.routeManager = mockRouteManager
+			engine.dnsServer = &dns.MockServer{}
 
 			defer func() {
 				exitErr := engine.Stop()
@@ -797,6 +801,7 @@ func TestEngine_MultiplePeers(t *testing.T) {
 				t.Errorf("unable to create the engine for peer %d with error %v", j, err)
 				return
 			}
+			engine.dnsServer = &dns.MockServer{}
 			mu.Lock()
 			defer mu.Unlock()
 			err = engine.Start()