[8.18](backport #8949) helm: define headless service for statefulset presets

[mergify]

What does this PR do?

This PR adds support for deploying Elastic Agent using StatefulSet by properly defining a serviceName, as required by the Kubernetes specification for StatefulSets.

Specifically, it:

• Introduces a headless service (clusterIP: None) per preset when mode: statefulset is used.
• Sets the serviceName field in the StatefulSet template to refer to the respective headless service.
• Provides a working example under examples/statefulset-preset/ showcasing a NetFlow integration running as a StatefulSet.

Why is it important?

StatefulSets in Kubernetes require the serviceName field to be specified to ensure stable network identities and persistent storage for each replica.

Checklist

• I have read and understood the pull request guidelines of this project.
• 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/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

None. This is an additive change that only affects users opting into StatefulSet mode. Existing other mode-based presets are unaffected.

How to test this PR locally

mage helm:renderExamples

Related issues

• Relates Elastic Agent StatefulSet does not support serviceName #8223

---

This is an automatic backport of pull request #8949 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

💛 Build succeeded, but was flaky

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cc @pkoutsovasilis