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/server_nonandroid.go

@@ -22,7 +22,7 @@ type defaultServerRouter struct {
 }
 
 func newServerRouter(ctx context.Context, wgInterface *iface.WGIface) (serverRouter, error) {
-	firewall, err := NewFirewall(ctx)
+	firewall, err := newFirewall(ctx)
 	if err != nil {
 		return nil, err
 	}