[Helm] Support ssl.verification_mode for standalone using Helm

[adeptr88]

Describe the enhancement:
Expose a Helm value to set ssl.verification_mode for the Elasticsearch output used by Elastic Agent in standalone mode. The chart should pass this value through to the rendered agent.yml.

Describe a specific use case for the enhancement or feature:
Our Elasticsearch uses an internal/self-signed CA (or traffic passes through TLS inspection). Without being able to set verification_mode, the Agent hits certificate validation errors. Having this knob in values.yaml lets us match the cluster’s PKI/inspection policy without post-render patches.

What is the definition of done?
values.yaml supports:

outputs:
  default:
    hosts: ["https://es01:9200"]
    ssl:
      verification_mode: full   # allowed: full | certificate | none

• The chart renders agent.yml with outputs.default.ssl.verification_mode exactly as provided.
• Input validation rejects values outside full|certificate|none with a clear error.
• README documents the option and its semantics.

[elasticmachine]

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

[adeptr88]

This seems already possible after all with.

outputs:
  default:
    ssl:
      verificationMode: [option]