Label MRs from Contractors

What does this MR do and why?

See https://gitlab.com/gitlab-org/plan/-/issues/766+

GitLab has started to utilize Temporary Service Providers. These appear as members of gitlab-org and have branch-creation rights in gitlab-org/gitlab but do not appear in the org chart.

This change adds a policy to automatically label Contractor contributions as such. In future this will allow:

• Changes to the review process specifically for Contractors; including additional rounds of review if necessary.
• Enumeration and identification of contractor contributions.

In order for their contributions to be labeled, the person will need to be added to the gitlab-org/contractors 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-with-a-dry-run on how to perform dry-runs.

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