charts: Add multi-port support for Service and Ingress backends

[Soli0222]

Summary

This PR adds multi-port support to the Helm chart so that sidecar containers added via deployment.extraContainers can be exposed through the same Headlamp Service and Ingress without maintaining a separate Service resource.

Related Issue

Fixes #5077

Changes

• Added service.extraServicePorts[] to append extra ports to the existing Headlamp Service (name / port / targetPort / protocol / nodePort).
• Added ingress.hosts[].paths[].backend.service.{name,port} so each Ingress path can override its backend. When backend is omitted the existing behavior (default Service / service.port) is preserved, so existing values files keep working unchanged.
• Updated values.yaml, values.schema.json, and README.md to document both fields.
• Added chart tests covering extraServicePorts and multi-backend Ingress (default fallback / port.name / port.number / templated name).

Steps to Test

1. bash charts/headlamp/tests/test.sh — all existing tests still pass (backward compatibility) and the two new test cases (service-extra-ports, ingress-multi-backend) pass.
2. Render the chart with the example values below and confirm the Service has both http and plugin ports, and the Ingress routes /plugin to port name plugin:

   service:
     extraServicePorts:
       - name: plugin
         port: 9090
         targetPort: plugin
   ingress:
     enabled: true
     hosts:
       - host: headlamp.example.com
         paths:
           - path: /
             type: Prefix
           - path: /plugin
             type: Prefix
             backend:
               service:
                 port:
                   name: plugin

3. helm lint charts/headlamp — clean.

Screenshots (if applicable)

N/A (chart-only change)

Notes for the Reviewer

• The new fields are purely additive; the default rendered output is unchanged, which the existing test suite confirms.
• The chart intentionally does not auto-create matching containerPort entries — users wiring a sidecar via extraContainers are expected to declare the port on the container side.
• The Gateway API path (httpRoute.rules) already accepts arbitrary backendRefs, so no template change was needed there; the README notes how to combine it with extraServicePorts.
• ingress.hosts[].paths[].backend.service.name is rendered through tpl so values like {{ .Release.Name }}-other work. Happy to drop this and treat the field as a literal string if you'd prefer to keep the trust boundary tighter.

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[linux-foundation-easycla]

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• ✅ login: Soli0222 / name: Soli0222 (637ca9f)

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[itvi-1234]

Hey @Soli0222 , Kindly sign the CNCF-CLA

[Soli0222]

Hi @itvi-1234, I just signed the CNCF-CLA. Could you please review it?