[8.12](backport #38162) PoC: use pipeline generator

[mergify]

Proposed commit message

• WHAT: pipeline migration from Jenkins to Buildkite using a pipeline generator based on Python.
• WHY: takes the concepts from [CI] Pipeline 2.0 for monorepos #20104 but for Buildkite

Tasks

• Enable when conditions.
  • GitHub comment - for individual stages and groups
  • GitHub labels - for individual stages
  • Changeset
  • Branches
• Use catalog to generate the pipeline itself, rather than calling a python script on a pipeline.yaml file.
• Support draft PRs.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

How to test this PR locally

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Logs

---

This is an automatic backport of pull request #38162 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 4b7467b has failed:

On branch mergify/bp/8.12/pr-38162
Your branch is up to date with 'origin/8.12'.

You are currently cherry-picking commit 4b7467b5c5.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .buildkite/auditbeat/scripts/unit-tests.sh
	new file:   .buildkite/buildkite.yml
	new file:   .buildkite/pipeline.py
	modified:   .buildkite/pipeline.yml
	new file:   .buildkite/scripts/generate_pipeline.sh
	new file:   auditbeat/buildkite.yml
	new file:   filebeat/buildkite.yml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .buildkite/hooks/pre-command

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[elasticmachine]

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