[8.16](backport #5855) [helm]: implement system integration as chart built-in

[mergify]

What does this PR do?

This PR introduces support for the system integration

Why is it important?

system integration is usually used in the context of k8s, thus having it as a built-in integration improves the user configuration experience

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/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

mage integration:kubernetesMatrix
```<hr>This is an automatic backport of pull request #5855 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of e6f281a has failed:

On branch mergify/bp/8.16/pr-5855
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit e6f281aa4d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   deploy/helm/elastic-agent/README.md
	modified:   deploy/helm/elastic-agent/examples/README.md
	new file:   deploy/helm/elastic-agent/examples/system-custom-auth-paths/README.md
	new file:   deploy/helm/elastic-agent/examples/system-custom-auth-paths/agent-system-values.yaml
	new file:   deploy/helm/elastic-agent/examples/system-custom-auth-paths/rendered/manifest.yaml
	modified:   deploy/helm/elastic-agent/templates/NOTES.txt
	modified:   deploy/helm/elastic-agent/templates/agent/_helpers.tpl
	new file:   deploy/helm/elastic-agent/templates/integrations/_system/_system.tpl
	new file:   deploy/helm/elastic-agent/templates/integrations/_system/_system_logs.tpl
	new file:   deploy/helm/elastic-agent/templates/integrations/_system/_system_metrics.tpl
	modified:   deploy/helm/elastic-agent/values.schema.json

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deploy/helm/elastic-agent/values.yaml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[pierrehilbert]

@pkoutsovasilis should we wait for 8.16.0 to be released or should we merge this one right now?

[pierrehilbert]

@pkoutsovasilis I let you fix the conflict as I want to be sure I won't do something bad :x

[pkoutsovasilis]

@pkoutsovasilis should we wait for 8.16.0 to be released or should we merge this one right now?

Being completely transparent here, I am not sure 😄 My logic is that this change is not a breaking one, the helm chart is not yet released so I don't see any immediate reason to wait for 8.16.0 release. But please do correct me if I miss anything 🙂

[ycombinator]

Yeah, I'm OK with adding this to 8.16.0. As @pkoutsovasilis said, we are still working out the release plan for the Helm chart and these changes don't add risk to the product (given how late we are in the 8.16.0 feature freeze cycle at this point).

[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