feat(slack): add users to threaded case on mention

[whitdog47]

This feature enhancement introduces the ability to add users to a threaded case in Slack when they are mentioned. This functionality aims to streamline collaboration by ensuring that all relevant participants are included in the case discussions.

Key Changes:

1. New Enum Values:

   • Added add_user and do_nothing actions to CaseNotificationActions in enums.py.

2. User Mention Handling:

   • Implemented handle_user_mention function in case/interactive.py to detect user mentions in messages.
   • Extracts mentioned users and checks if they are part of the case.
   • Sends a private message to the user who mentioned others, offering to add the mentioned users to the case.

3. User Addition Workflow:

   • Added add_users_to_case function to handle the addition of users to a case when the "Add Them" button is clicked.
   • Utilizes AddUserMetadata to manage user data and case context.

4. Middleware Enhancements:

   • Introduced add_user_middleware in middleware.py to process user addition requests.
   • Ensures the correct context is passed for user addition actions.

5. UI Components:

   • Created UI blocks for user notification and action buttons in Slack messages.
   • Utilized DefaultBlockIds for consistent block identification.

6. Model Updates:

   • Added AddUserMetadata class in models.py to encapsulate user and case metadata.