[8.16](backport #6345) [helm] fleet mode fixes

[mergify]

What does this PR do?

This PR introduces several fixes to the Elastic Agent Helm chart regarding Fleet Mode deployments:

• Fleet mode now includes the necessary Kubernetes changes (e.g., volume mounts, resource requests) for enabled integrations.
• Leader election is enabled by default for Fleet mode unless explicitly disabled by the user in the agent preset.
• Kubernetes integration is now enabled by default.
• Sets hostNetwork=true for perNode (Daemonset) agent preset, used by kubernetes and system integrations

Why is it important?

These changes are critical as they address bugs 🙂

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

N/A

How to test this PR locally

• You can see in the ./deploy/helm/examples/fleet-managed/rendered/manifest.yaml that now fleet-managed agent has the appropriate volume mounts and leader election enabled.
• Follow this example and you should be able to see container logs correctly

Related issues

• Closes [Helm] Enable leader election by default for Fleet Managed agent installation with perNode preset #6284
• Closes [Helm] Chart does not mount /var/log/containers/ on Elastic-Agent pod, thus no logs are ingested #6204
• Closes [Helm] Missing hostNetwork: true for managed agent in perNode preset #6324

---

This is an automatic backport of pull request #6345 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 0d94ead has failed:

On branch mergify/bp/8.16/pr-6345
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit 0d94ead04.
  (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/eck/rendered/manifest.yaml
	modified:   deploy/helm/elastic-agent/examples/fleet-managed/README.md
	modified:   deploy/helm/elastic-agent/examples/fleet-managed/fleet-values.yaml
	modified:   deploy/helm/elastic-agent/examples/fleet-managed/rendered/manifest.yaml
	modified:   deploy/helm/elastic-agent/examples/kubernetes-default/rendered/manifest.yaml
	modified:   deploy/helm/elastic-agent/examples/kubernetes-only-logs/rendered/manifest.yaml
	modified:   deploy/helm/elastic-agent/examples/system-custom-auth-paths/agent-system-values.yaml
	modified:   deploy/helm/elastic-agent/examples/system-custom-auth-paths/rendered/manifest.yaml
	modified:   deploy/helm/elastic-agent/examples/user-service-account/rendered/manifest.yaml
	modified:   deploy/helm/elastic-agent/templates/agent/_helpers.tpl
	modified:   deploy/helm/elastic-agent/values.yaml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deploy/helm/elastic-agent/examples/kubernetes-hints-autodiscover/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/examples/multiple-integrations/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/templates/integrations/_kubernetes/_preset_pernode.tpl

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

[mergify]

This pull request has not been merged yet. Could you please review and merge it @pkoutsovasilis? 🙏

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
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See analysis details on SonarQube