[Integration] [Slack] Introduce Slack integration for channel & user sync

[devin-ai-integration]

Slack Integration for Port Ocean

This PR introduces a new Slack integration for Port Ocean that enables synchronization of Slack channels and users with Port.

Features

• Synchronize Slack channels and their properties
• Synchronize Slack users (with PII data masking)
• Track channel membership
• Real-time updates via webhooks
• Rate limit handling
• Comprehensive test coverage

Testing

• Unit tests cover:
  • Initial sync functionality
  • Rate limit handling
  • Webhook event processing
  • Error scenarios
  • Pagination handling
  • Channel member synchronization

Documentation

• Added README with setup instructions
• Included configuration examples
• Documented PII data handling

Link to Devin run: https://app.devin.ai/sessions/61397cc1a78e4967832be1f9708b4cd6

[devin-ai-integration]

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