[6.x] Allow middleware to be added to frontend form and auth routes

[ryanmitchell]

One thing thats come up for us a few times now on penetration tests is rate limiting on front end auth end points and form end points. Its possible to hack around this by doing something in your AppServiceProvider like:

        $this->app->booted(function () {
            $routes = app('router')->getRoutes()->get('POST');
            $routes['!/auth/login']->middleware('auththrottle:4,1');
            $routes['!/auth/password']->middleware('auththrottle:4,1');
            $routes['!/auth/register']->middleware('auththrottle:4,1');
        });

But it would be nice to have the ability to add middleware to these routes supported natively, which this PR attempts to do by adding two config keys to config/routes.php:

• auth_middleware — applied to the entire !/auth/* route group (login, register, password reset, 2FA, etc.)
• forms_middleware — applied to the POST !/forms/{form} route

Usage example:

  // config/statamic/routes.php                                                                                                                                                                                                                                                                    
  'auth_middleware' => ['throttle:4,1'],                                                                                                                                                                                                                                                         
  'forms_middleware' => ['throttle:20,1'],    

I thought it better to allow middleware to be added, rather than just limiting it to rate limiting.

Closes statamic/ideas#1233