ci: deploy snapshots using GitHub actions in conjunction with Buildkite

[v1v]

What does this PR do?

Use GitHub actions and Buildkite to run the snapshot releases.

GitHub actions will be the only interface that the team will need to interact with, and there is a new GitHub action in charge of triggering the snapshot release in Buildkite.

This particular approach has been validated and done in some other projects, see elastic/ecs-logging-java#197

Caveats

Only supported for snapshot releases. Hence, the release automation will be done in a follow up.
Duplicated .ci/settings.xml to avoid messing around with the release automation.

[apmmachine]

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• Start Time: 2023-03-23T10:39:14.868+0000

• Duration: 12 min 25 sec

❕ Flaky test report

No test was executed to be analysed.

🤖 GitHub comments

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To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run benchmark tests : Run the benchmark tests.

• run jdk compatibility tests : Run the JDK Compatibility tests.

• run integration tests : Run the Agent Integration tests.

• run end-to-end tests : Run the APM-ITs.

• run windows tests : Build & tests on windows.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)