Add support in UniqueComment for PipelineEvents

Context

UniqueComment assumes an webhook event is associated to a noteable resource (e.g. issue, MR, Epic, something you can write a comment into).

We break this assumption when we are trying to post a comment in response to a PipelineEvent Webhook.

This MR ensures UniqueComment doesn't assume anything from the received webhook event.

We can now pass any noteable resource in which we want to check whether a UniqueComment was used.

What does this MR do?

• Add support in UniqueComment for PipelineEvents
• Comments in the MR associated to a PipelineEvent in the NudgeForMissingAutoMerge processor

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-policies-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