Add Slack Get*Settings to use typed struct

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Add Slack integration, and use the v2.0 typed struct pattern.

Instructions

  1. Investigation: Verify the properties JSON structure via the GitLab API.
  2. Struct Definition: Implement SlackIntegration (embedding Integration) and SlackIntegrationProperties.
  3. Add: Method signatures in the IntegrationsService.
  4. Validation: Update mocks and add relevant unit tests.

Parent Epic: #2198

Edited by 🤖 GitLab Bot 🤖