Introduce SlackNotifier to handle Slack notification for pipeline failures

What does this MR do and why?

Add support for Slack notification in pipeline failure configuration classes, and a Triage::PipelineFailure::SlackNotifier class to post messages to Slack based on these configuration classes.

This MR includes a few refactoring commits to prepare for the new processor to come: !2124 (merged).

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review

Closes #1294 (closed), part of gitlab-org&10172 (closed).