Support standard actions at item playback end

[waliid]

Description

This PR allows us to add a way to change the behavior of an item when it's ended.

Changes made

• The property actionAtItemEnd has been introduced.
• A new analytics session is created when an item is ended, even if it's paused.
• The demo app has been updated:
  • The stories showcase is paused at the end.
  • An opt-in option has been added in the opt-in showcase.

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).