buildkite: run DRA if any changes

[v1v]

Proposed commit message

Run the DRA pipelines when there are any changes in some of the files related to the DRA.

This should help with running https://github.com/elastic/beats/pull/44701#issuecomment-2977729334 easily

So far, I decided to run for SNAPSHOTs and use BUILDKITE_PULL_REQUEST_BASE_BRANCH as the DRA_BRANCH.

Finally, i decided to run dry-run mode and without any slack notifications - so we avoid massive reporting for 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.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

In the CI only

Related issues

Use cases

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