[8.x](backport #5643) [Kubernetes] Add kustomize template for hints auto discover

[mergify]

What does this PR do?

This PR

1. adds a Kustomize template to enable hints based auto discovery by default in standalone elastic agent.
   It adds a init container block in elastic agent daemonset.
   For all the rest resources needed for the deployment of elastic agent it relies on the resources from the default kustomize templates.
2. Updates the volume mount of the init container and adds a shared volume between init and main container for storing the input templates. This allows for removal of root privileges in init container which can be a security vulnerability.

Why is it important?

To enable users of the Observability Kubernetes quickstart to make use of hints auto discovery without the need of configuring and redeploying their manifests.

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

Follow instructions of https://github.com/elastic/elastic-agent/tree/main/deploy/kubernetes/elastic-agent-kustomize/default#remote-usage-of-kustomize-templates

Use https://github.com/MichaelKatsoulis/elastic-agent/deploy/kubernetes/elastic-agent-kustomize/ksm-hints/elastic-agent-standalone\?ref\=kustomize_templates_hints_autodiscover for the kustomization file link.

Related issues

• Closes https://github.com/elastic/ingest-dev/issues/3760

---

This is an automatic backport of pull request #5643 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of eedce01 has failed:

On branch mergify/bp/8.x/pr-5643
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit eedce01997.
  (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:
	new file:   changelog/fragments/1727790202-kustomize-templates-with-hints.yaml
	modified:   deploy/kubernetes/Makefile
	modified:   deploy/kubernetes/elastic-agent-kustomize/default/README.md
	new file:   deploy/kubernetes/elastic-agent-kustomize/ksm-hints/elastic-agent-standalone/elastic-agent-standalone-daemonset-configmap.yaml
	new file:   deploy/kubernetes/elastic-agent-kustomize/ksm-hints/elastic-agent-standalone/kustomization.yaml
	new file:   deploy/kubernetes/elastic-agent-kustomize/ksm-hints/elastic-agent-standalone/kustomization.yaml.original
	modified:   deploy/kubernetes/elastic-agent-standalone/elastic-agent-standalone-daemonset.yaml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deploy/kubernetes/elastic-agent-kustomize/default/elastic-agent-standalone/base/elastic-agent-standalone-daemonset.yaml
	both modified:   deploy/kubernetes/elastic-agent-kustomize/ksm-autosharding/elastic-agent-standalone/base/elastic-agent-standalone-daemonset.yaml
	both modified:   deploy/kubernetes/elastic-agent-kustomize/ksm-autosharding/elastic-agent-standalone/extra/elastic-agent-standalone-statefulset.yaml
	both modified:   deploy/kubernetes/elastic-agent-standalone-kubernetes.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

[mergify]

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

[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