Add Buildkite trigger context annotation

[oakrizan]

Proposed commit message

Added Buildkite annotation, for scenarios when build is triggered by GH comment without code changes

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.

Related issues

• Relates: https://github.com/elastic/ingest-dev/issues/3041

Logs

Buildkite builds: https://buildkite.com/elastic/beats/builds?branch=oakrizan%3Aadd-triggering-reason

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @oakrizan? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[oakrizan]

/test

[oakrizan]

/test

[oakrizan]

/test

[oakrizan]

/test