Add slack integration support

Closes #2201 (closed)

As discussed here !2670 (merged), integrations should have dedicated structs that was ment for the `release-client-2.0`.

But, for the integrations that do not exist, we will target the main Instead, as discussed here: !2681 (closed)

This is what the API returns

curl -s -X GET "${API_URL}/groups/${GROUP_ID}/integrations/slack" \
  -H "PRIVATE-TOKEN: ${GITLAB_TOKEN}" \
  -H "Content-Type: application/json" | jq .

{
  "id": 53,
  "title": "Slack notifications",
  "slug": "slack",
  "created_at": "2026-01-23T17:52:37.490Z",
  "updated_at": "2026-01-23T17:52:37.490Z",
  "active": true,
  "commit_events": true,
  "push_events": true,
  "issues_events": true,
  "incident_events": false,
  "alert_events": true,
  "confidential_issues_events": true,
  "merge_requests_events": true,
  "tag_push_events": true,
  "deployment_events": false,
  "note_events": true,
  "confidential_note_events": true,
  "pipeline_events": true,
  "wiki_page_events": true,
  "job_events": true,
  "comment_on_event_enabled": true,
  "inherited": false,
  "properties": {
    "username": "testuser",
    "channel": "general",
    "notify_only_broken_pipelines": true,
    "branches_to_be_notified": "default",
    "labels_to_be_notified": "bug",
    "labels_to_be_notified_behavior": "match_any",
    "push_channel": "push-channel",
    "issue_channel": "issue-channel",
    "confidential_issue_channel": "conf-issue-channel",
    "merge_request_channel": "mr-channel",
    "note_channel": "note-channel",
    "confidential_note_channel": "conf-note-channel",
    "tag_push_channel": "tag-push-channel",
    "pipeline_channel": "pipeline-channel",
    "wiki_page_channel": "wiki-channel",
    "deployment_channel": "deploy-channel",
    "incident_channel": "incident-channel",
    "alert_channel": "alert-channel",
    "group_mention_channel": "mention-channel",
    "group_confidential_mention_channel": "conf-mention-channel"
  }
}

Part of: Epic #2198 - Refactor Integration Get*Settings Breaking Change: New methods added to IntegrationsServiceInterface