Possibility to set maintenance page just for the storefront

[vitek-rostislav]

Describe the expected outcome of your requested feature

Sometimes, it might become handy to have the possibility to turn on the maintenance page just for the storefront and keep the administration part available. We needed this during the launch of a new project.

Additional context

I suggest adding a new ENV variable that would enable this behavior, the ENV could be then used in FrontendApiController and if set, it could just return new Response('', Response::HTTP_SERVICE_UNAVAILABLE);