Add support for Group Mattermost integrations

Hamza Hassanainrequested to merge
gitlab-community/gitlab-org/api/client-go:feat/group-mattermost-integration into main

Closes #2196 (closed)

This merge request adds support for managing Group Mattermost integrations through the GitLab API client.

What's New

Mattermost Notifications Integration:

  • Retrieve integration settings for a group
  • Create or update integration configuration with support for webhooks, channels, and event notifications
  • Delete integration from a group
  • Channel-specific routing for different event types

Mattermost Slash Commands Integration:

  • Retrieve slash command configuration
  • Set up or update slash command tokens
  • Remove slash command integration

Technical Details

  • 6 new API methods added to the IntegrationsService
  • 4 new data structures for request/response handling
  • 6 comprehensive unit tests covering all CRUD operations
  • Interface definitions updated with new method signatures
  • Mock implementations regenerated for testing
  • CHANGELOG updated to document the additions

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