[8.x](backport #5854) [helm]: implement support for defining image pull secrets

[mergify]

What does this PR do?

This PR that adds support for defining pullImageSecrets support in the chart

Why is it important?

Users mights need to deploy an agent image from a private repository

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

mage integration:kubernetesMatrix

---

This is an automatic backport of pull request #5854 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 053b320 has failed:

On branch mergify/bp/8.x/pr-5854
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 053b3200fc.
  (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/kubernetes-default/README.md
	modified:   deploy/helm/elastic-agent/templates/agent/eck/_pod_template.tpl
	modified:   deploy/helm/elastic-agent/templates/agent/k8s/_pod_template.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

[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