Make IncidentCreator and SlackNotifier take a Config class

What does this MR do and why?

This adds config for all the pipeline failure cases we want to port from gitlab-org/gitlab pipeline config.

This also refactors the IncidentCreator and SlackNotifier classes to take a Config object, which reduces the responsibilities of these classes, and allow for more composition.

I also opened gitlab-org/gitlab!116467 (merged) which should be merged just before we merge this MR.

Expected impact & dry-runs

I expect no behavior change as this MR should make sure the incident creation and notifications are now done by triage-ops.

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 #1298 (closed)