[Helm] Support outputs.default.hosts (list) for standalone using Helm

[adeptr88]

Describe the enhancement:
Expose a hosts list for the Elasticsearch output in Helm values (standalone Agent), not just a single url string. The chart should render outputs.default.hosts: [...] into agent.yml (backward-compatible with today’s url).

Describe a specific use case for the enhancement or feature:
On-prem clusters without a load balancer need multiple ES HTTP endpoints for failover/HA. With only a single url, one node outage breaks ingest.

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

outputs:
  default:
    hosts:
      - https://es01:9200
      - https://es02:9200
      - https://es03:9200

• Chart renders agent.yml with outputs.default.hosts (if hosts set), and continues to support url (mutually exclusive; emit a clear validation error if both are set).
• README documents the new option with examples.

[elasticmachine]

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