Add default firewall rule to allow netbird traffic (#1056)

Add a default firewall rule to allow netbird traffic to be handled 
by the access control managers.

Userspace manager behavior:
- When running on Windows, a default rule is add on Windows firewall
- For Linux, we are using one of the Kernel managers to add a single rule
- This PR doesn't handle macOS

Kernel manager behavior:
- For NFtables, if there is a filter table, an INPUT rule is added
- Iptables follows the previous flow if running on kernel mode. If running 
on userspace mode, it adds a single rule for INPUT and OUTPUT chains

A new checkerFW package has been introduced to consolidate checks across
route and access control managers.
It supports a new environment variable to skip nftables and allow iptables tests

client/internal/routemanager/manager.go

@@ -36,7 +36,7 @@ type DefaultManager struct {
 
 // NewManager returns a new route manager
 func NewManager(ctx context.Context, pubKey string, wgInterface *iface.WGIface, statusRecorder *peer.Status, initialRoutes []*route.Route) *DefaultManager {
-	serverRouter, err := newServerRouter(ctx, wgInterface)
+	srvRouter, err := newServerRouter(ctx, wgInterface)
 	if err != nil {
 		log.Errorf("server router is not supported: %s", err)
 	}
@@ -46,7 +46,7 @@ func NewManager(ctx context.Context, pubKey string, wgInterface *iface.WGIface,
 		ctx:            mCTX,
 		stop:           cancel,
 		clientNetworks: make(map[string]*clientNetwork),
-		serverRouter:   serverRouter,
+		serverRouter:   srvRouter,
 		statusRecorder: statusRecorder,
 		wgInterface:    wgInterface,
 		pubKey:         pubKey,