[8.14](backport #39792) [CI] Agentbeat PR pipeline

[mergify[bot]]

Proposed commit message

Needs https://github.com/elastic/beats/pull/39794

Creates an agentbeat CI pipeline that packages agentbeat on pull requests. Triggers when following projects are changed:

• auditbeat
• filebeat
• heartbeat
• metricbeat
• osquerybeat
• packetbeat
• libbeat
• testing

The agentbeat pipeline is also triggered on dev-tools, go.mod and other service files changes

Need a better solution!

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

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This is an automatic backport of pull request #39792 done by Mergify.