Move Elastic agent pipeline config to main

[pazone]

Proposed commit message

Moved xpack/elastic-agent 7.17 buildkite pipeline metadata to main. Because backstage is configured to read only the main branch.

Initial pipeline configuration should be merged to main. Then further xpack/elastic-agent pipeline migration will be possible

[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @pazone? 🙏.
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-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2024-01-31T12:23:05.235+0000

• Duration: 14 min 37 sec

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Passed	3
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Total	3

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