WIP: Feature/erp5 smart backend

A backend smart enough, that can serve correctly with frontend without having knowledge about zope type.

Requirements:

• by default the non-path (/) setup is provided
  • /VirtualHostBase/{frontend-schema}/{frontend-hostname}:{frontend-port}/VirtualHostRoot/{frontend-path}
• default-path from caddy-frontend entry point has to be implemented here, as this is special case to handle the path, possibly exposed on some specific ip/port combination
  • /VirtualHostBase/{frontend-schema}/{frontend-hostname}:{frontend-port}/{backend-path}/VirtualHostRoot/{frontend-path}
• virtualhostroot-http-port and virtualhostroot-https-port shall be extracted from request itself (trust that frontend gives proper information in the URL)

TODOs:

• follow https://lab.nexedi.com/nexedi/slapos/merge_requests/1070#note_169765