Introduce classes to create incidents from pipeline events

What does this MR do and why?

Introduces classes to hold configuration related to pipeline failure in different contexts (master / stable branches, child review app pipelines), and a Triage::PipelineFailure::IncidentCreator class to create incidents based on these configuration classes.

Expected impact & dry-runs

This MR doesn't introduce any processor yet, so I expect no behavior change.

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 #1292 (closed), part of gitlab-org&10172 (closed).