Draft: Separate Event class

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What does this MR do and why?

Describe in detail what your merge request does and why.

Separate Event class

The Event class is building several kinds of events based on the object_kind field in the webhook payload (e.g. IncidentEvent, IssueEvent, ...). This file contains all the implementations of the events.

As a result, the triage/triage/event.rb file is close to 1000 lines and should be separated for readability.

Related to #1591

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

Closes #1591