Add support for pre-created Kubernetes secrets in Helm chart

[dpacheconr]

Summary

Add support for using pre-created Kubernetes secrets with the Helm chart via new customSecretName and customSecretKey values. This enables integration with external secret management tools like External Secrets Operator, Sealed Secrets, and Vault.

Changes

• Add customSecretName value to reference an existing secret instead of creating one
• Add customSecretKey value to customize the key name in external secrets (defaults to personalAPIKey)
• Make secret creation conditional based on customSecretName
• Add helper functions to determine secret name and key at runtime
• Update deployment to use configurable secret references
• Update README with documentation and usage examples
• Remove deprecated engine: gotpl field from Chart.yaml for Helm v3 compliance
• Bump chart version to 1.18.0

Backward Compatibility

Fully backward compatible - existing installations continue to work without changes. The default behavior (creating a secret from personalAPIKey) is preserved when customSecretName is not set.

Testing

• Tested chart linting with both default and custom secret scenarios
• Verified Helm template rendering for all combinations
• Tested on live Minikube cluster:
  • Scenario 1: Default personalAPIKey creates secret automatically
  • Scenario 2: Custom secret name skips secret creation and uses external secret

Documentation

Updated README.md with usage examples for External Secrets Operator, Sealed Secrets, Vault, and manual secret creation.