[R5] Create an Item data model & an ItemCollection class, and move/refactor List Items API into the class

[wl2484]

As a developer I need to create (1) an Item data model and (2) ItemCollection class and move/refactor List Items API into the class So that List Items API swagger doc would be available on the /apidocs page

Details and Assumptions

• https://github.com/nyu-devops/lab-flask-restplus-swagger/blob/master/service/routes.py#L208

  Acceptance Criteria

  TBA

  Given [some context]
  When [action taken]
  Then [outcome observed]

[momo100110]

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