Add Mattermost integration

[Djiit]

This issue is to add Mattermost integration support to gong.

Overview

The Mattermost integration should allow sending notifications to Mattermost channels about pending PR reviews.

Implementation Details

1. Create a new package in internal/integrations/mattermost
2. Implement the Mattermost integration interface
3. Add support for Mattermost webhook URLs
4. Add configuration parameters for Mattermost notifications

Acceptance Criteria

• Mattermost notifications can be configured globally or per rule
• Mattermost integration can be used alongside other integrations (Slack, Comment, etc.)
• The integration is properly documented in the docs
• Unit tests are written for the implementation
• Update examples in configuration documentation

References

See existing integrations like Slack integration for a good example to follow.

Labels: integration, help wanted