[8.x](backport #6004) [helm] fix disabling cluster role and service account

[mergify]

What does this PR do?

This PR resolves two key issues:

1. It enables disabling cluster role creation without needing to specify an existing role name.
2. It allows falling back to the default service account by disabling service account creation without specifying an existing one.

Additionally, it updates the nginx custom integration example to demonstrate how to deploy an elastic agent with Kubernetes providers, leader election, cluster role, and service account creation all disabled.

Why is it important?

These changes are essential for improving the configuration flexibility of the deployment process. By allowing cluster role creation to be disabled without requiring the specification of an existing role name, users gain the ability to simplify permissions management. This is particularly useful in environments where pre-configured roles or restricted permissions are mandated.

Furthermore, the option to fall back to the default service account without creating a new one or specifying an existing account name enhances compatibility with Kubernetes setups that have stringent service account policies or rely on default configurations. This ensures smoother integration and reduces the complexity for users deploying the elastic agent in a wide variety of Kubernetes environments.

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

Follow the updated nginx custom integration example

Related issues

N/A

---

This is an automatic backport of pull request #6004 done by Mergify.

[mergify]

Cherry-pick of a0111f1 has failed:

On branch mergify/bp/8.x/pr-6004
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit a0111f1789.
  (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/examples/nginx-custom-integration/README.md
	modified:   deploy/helm/elastic-agent/examples/nginx-custom-integration/agent-nginx-values.yaml
	modified:   deploy/helm/elastic-agent/templates/agent/cluster-role-binding.yaml
	modified:   deploy/helm/elastic-agent/templates/agent/eck/_pod_template.yaml
	modified:   deploy/helm/elastic-agent/templates/agent/k8s/_pod_template.yaml
	modified:   deploy/helm/elastic-agent/values.schema.json

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deploy/helm/elastic-agent/examples/nginx-custom-integration/rendered/manifest.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

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[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