Add option to get stories by folder per store view

[rossmc]

Adds a new Sys Config option for the top level folder in-which Storyblok stories are contained.

May be used for:

• multi-language Magento installs for folder-level translations. https://www.storyblok.com/docs/guide/in-depth/internationalization#folder-level-translation
• multi website / store view Magento installs to manage all stories from one space.