enhancement(slack): adding slash command and button to manually engage user for mfa prompt

[whitdog47]

This pull request introduces a new feature to the Dispatch Slack plugin, allowing users to manually engage participants in a case for Multi-Factor Authentication (MFA) verification. The enhancement includes the addition of a slash command and a button to facilitate this process.

Key Changes:

1. Interactive Command and Button:

   • Implemented handle_engage_user_command in interactive.py to handle the new /dispatch-engage-user Slack command.
   • Added a button labeled "User MFA" to the case message actions, allowing users to initiate an MFA prompt directly from the Slack interface.

2. Modal and Engagement Handling:

   • Created a modal for engaging users via MFA, allowing customization of the engagement message and selection of the participant.
   • Developed create_manual_engagement_message in messages.py to generate the engagement message structure.
     

3. Configuration Updates:

   • Added slack_command_engage_user to SlackConversationConfiguration in config.py to define the new command string for engaging users.

This enhancement aims to improve the security and flexibility of user engagement within the Dispatch Slack integration, providing teams with more robust tools for managing case-related interactions.