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

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.

Start with the chart's values.yaml and the templates that render agent.yml, then check the README examples. Verify how the existing default url is rendered and how values are validated. Done means hosts renders when set, url remains supported, both together produce a clear validation error, and the README documents the new list option.