[CI] Add trigger job for baseline job

[brianseeders]

@spalger requested this. This is a job to make sure that each individual commit has a corresponding baseline build. Jenkins doesn't really have this functionality built-in, so if multiple commits are pushed in a short window, some of them could be skipped without a job like this.

For each branch:

• Iterate through commits using the Github api, collecting ones that have not yet been built/started (this is tracked via the commit status with kibana-ci-baseline context). Stop at the first one that's already been triggered.

For each commit that needs a build:

• Create a Github commit status of 'pending'
• Trigger the baseline job, and immediately move on
• When the baseline job finishes, it will set a commit status for 'success' or 'error'

There will be a job set up that:

• Will not run concurrently
• Trigger on commit
• Also trigger on a timer, as a backup

[spalger]

@elasticmachine merge upstream

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 5ab3dac

Build metrics

✅ unchanged

History

• 💛 Build #54849 was flaky 50fc77b

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[spalger]

Merged in a5c9c4e#diff-5128ec7a50ab10871f37b6f4bb36e2f2