Add PipelineTierTransitions processor (in sandbox mode)

Context

Related to #1539 (closed).

I was working on changing the current process in !2838 (diffs), and I thought it would be a good thing to test the new logic before we roll it out to the company.

I'll therefore split the original MR into two parts:

1. Introduce the PipelineTierTransitions processor (this MR)
2. Replace the current process with PipelineTierTransitions (this will be the "future" !2838 (merged) at the time of writing)

What does this MR do and why?

Introduces the PipelineTierTransitions processor in sandbox mode.

We're now creating new pipelines based on tiers information, and not based on the pipeline:mr-approved label (even though pipeline:mr-approved is still the label we use to include a lot more jobs).

Once we test this processor and validates that it works as expected, we can review the follow-up MR to replace the existing process.

Expected impact & dry-runs

None on the current processes, as we run in sandbox mode.

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