[EAK-558] Etoolbox Lists: add simple json view for list items

[liubou-masiuk]

ACS Commons has the functionality to render a simple JSON representation of all list items (by opening a list page with .list.json extension). It would be nice to add a similar view for Etoolbox Lists. It would simplify migration from ACS Lists to Etoolbox Lists for cases that use the data from the list in JSON format

Example of ACS Commons list JSON output