[8.x](backport #5992) [helm] add envFrom in agent preset

[mergify]

What does this PR do?

This PR adds support for specifying envFrom in the pod template of the Helm chart. By introducing this feature, users can configure environment variables from ConfigMaps or Secrets more flexibly and efficiently. The change modifies the pod template to include a new section that allows injecting envFrom values, providing an enhanced way to handle environment settings.

Why is it important?

This enhancement is useful for users who need to manage environment variables dynamically through ConfigMaps or Secrets e.g. through ExternalSecrets.

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

1. create a k8s secret with the name $secretName
2. follow the kubernetes example and add this param --set 'agent.presets.perNode.envFrom[0].secretRef.name'=$secretName

Related issues

N/A

---

This is an automatic backport of pull request #5992 done by [Mergify](https://mergify.com).

[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

[elasticmachine]

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