Support client default routes for Linux (#1667)

All routes are now installed in a custom netbird routing table.
Management and wireguard traffic is now marked with a custom fwmark.
When the mark is present the traffic is routed via the main routing table, bypassing the VPN.
When the mark is absent the traffic is routed via the netbird routing table, if:
- there's no match in the main routing table
- it would match the default route in the routing table

IPv6 traffic is blocked when a default route IPv4 route is configured to avoid leakage.

client/internal/routemanager/manager_test.go

@@ -28,13 +28,14 @@ const remotePeerKey2 = "remote1"
 
 func TestManagerUpdateRoutes(t *testing.T) {
 	testCases := []struct {
-		name                          string
-		inputInitRoutes               []*route.Route
-		inputRoutes                   []*route.Route
-		inputSerial                   uint64
-		removeSrvRouter               bool
-		serverRoutesExpected          int
-		clientNetworkWatchersExpected int
+		name                               string
+		inputInitRoutes                    []*route.Route
+		inputRoutes                        []*route.Route
+		inputSerial                        uint64
+		removeSrvRouter                    bool
+		serverRoutesExpected               int
+		clientNetworkWatchersExpected      int
+		clientNetworkWatchersExpectedLinux int
 	}{
 		{
 			name:            "Should create 2 client networks",
@@ -200,8 +201,9 @@ func TestManagerUpdateRoutes(t *testing.T) {
 					Enabled:     true,
 				},
 			},
-			inputSerial:                   1,
-			clientNetworkWatchersExpected: 0,
+			inputSerial:                        1,
+			clientNetworkWatchersExpected:      0,
+			clientNetworkWatchersExpectedLinux: 1,
 		},
 		{
 			name: "Remove 1 Client Route",
@@ -415,6 +417,8 @@ func TestManagerUpdateRoutes(t *testing.T) {
 			statusRecorder := peer.NewRecorder("https://mgm")
 			ctx := context.TODO()
 			routeManager := NewManager(ctx, localPeerKey, wgInterface, statusRecorder, nil)
+			err = routeManager.Init()
+			require.NoError(t, err, "should init route manager")
 			defer routeManager.Stop()
 
 			if testCase.removeSrvRouter {
@@ -429,7 +433,11 @@ func TestManagerUpdateRoutes(t *testing.T) {
 			err = routeManager.UpdateRoutes(testCase.inputSerial+uint64(len(testCase.inputInitRoutes)), testCase.inputRoutes)
 			require.NoError(t, err, "should update routes")
 
-			require.Len(t, routeManager.clientNetworks, testCase.clientNetworkWatchersExpected, "client networks size should match")
+			expectedWatchers := testCase.clientNetworkWatchersExpected
+			if runtime.GOOS == "linux" && testCase.clientNetworkWatchersExpectedLinux != 0 {
+				expectedWatchers = testCase.clientNetworkWatchersExpectedLinux
+			}
+			require.Len(t, routeManager.clientNetworks, expectedWatchers, "client networks size should match")
 
 			if runtime.GOOS == "linux" && routeManager.serverRouter != nil {
 				sr := routeManager.serverRouter.(*defaultServerRouter)