Add service proxy support and enable Helm integration in headlamp

[shahvrushali22]

Summary

This PR adds in-cluster service proxy support to Headlamp, enables Helm operations with authenticated access, and refactors token validation for consistency. It also ensures namespace scoping in Helm charts and enables secure HTTPS-based readiness and liveness probes.

Changes

• Added serviceproxy backend module to enable authenticated in-cluster service access.
• Registered new route /clusters/{cluster}/serviceproxy/{namespace}/{name}.
• Refactored checkHeadlampBackendToken into a method for consistency.
• Enabled Helm support via --enable-helm flag and validated tokens using SelfSubjectReview.
• Forwarded Bearer tokens for secure Helm operations.
• Updated Helm chart templates:
• Added namespace scoping to all resources.
• Enabled HTTPS for liveness/readiness probes and services.
• Made Helm feature toggleable via values.yaml.
• Added config test for enableHelm flag and updated expected chart templates.

Steps to Test

1. Install Headlamp with Helm support enabled
2. Test in-cluster service proxy route
3. Test Helm integration in the UI

[k8s-ci-robot]

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