feat(ec): lint helm charts declared in EC Config extensions

[emosbaugh]

Summary

Extends the existing helm chart archive validation to also recognize helm charts declared in EmbeddedCluster Config manifests (extensions.helmCharts[].chart.name / chartVersion).

Changes

• Introduced a chartIdentity interface so the archive-matching logic works for both:
  • kots.io/v1beta1 HelmChart custom resources
  • embeddedcluster.replicated.com/v1beta1 Config with spec.extensions.helmCharts
• Added findAllECConfigHelmCharts() to walk EC Config manifests and extract chart name/version pairs.
• Updated archiveForHelmChartExists() and helmChartForArchiveExists() to accept the new interface.
• Added unit tests in pkg/kots/lint_test.go and integration tests in test/integration/integration_test.go covering:
  • EC Config helm chart with matching archive (no errors)
  • EC Config helm chart missing archive (helm-archive-missing)
  • Archive present with only EC Config entry (no helm-chart-missing)

[emosbaugh]

[sc-137591]