Adds support for fullstack engineering managers

What does this MR do and why?

Our job roles for engineering managers includes frontend, backend, fullstack, and engineering manager (generic). This MR adds support for fullstack engineering managers. The support includes scenarios where there are only fullstack mangers (as of writing, threat insights is the only example of this) as well as falling back to fullstack managers when we're selecting an engineering manager and there isn't a more specific person described in the team.

The reports have been updated to include the fullstack manager by default as have our test cases.

This MR should result in the @nmccorrison and/or @kniechajewicz receiving pings on their reports which would not have been happening before. This MR should result in other fullstack engineering managers being tagged on reports where there are other engineering managers in their group.

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

/cc @nmccorrison @kniechajewicz