Introduce a new 'Workflow::MasterBrokenLabel' processor

What does this MR do and why?

This new processor does two things:

1. Adds the pipeline:expedite-master-fixing labelto MRs that have themaster:brokenormaster:foss-broken` label
2. Removes pipeline:expedite-master-fixing from MRs that don't have the master:broken or master:foss-broken label

This relates to the process at https://about.gitlab.com/handbook/engineering/workflow/#resolution-of-broken-master.

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 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