Fix existingSecrets implementation in unleash-edge chart

Description

This PR fixes the implementation of existingSecrets in the unleash-edge Helm chart which currently causes Kubernetes validation errors when deploying with secret configurations. The issue is documented in unleash#9111.

Changes

1. Modified deploment.yaml default to use envFrom to load secrets

{{- if Values.existingSecrets }}
envFrom:
  - secretRef:
      name: {{ .Values.existingSecrets }}
{{- end }}

2. Updated the deployment template to use envFrom instead of trying to merge secrets into env section, which prevents validation errors with environment variables.

Testing

• Deployed chart with empty secrets configuration (works)
• Deployed chart with secrets configured:

existingSecrets: custom-user-secret  # references existing secret

• Verified deployment completes without validation errors
• Verified environment variables are properly set in the container

Additional Notes

• This is a breaking change as it modifies how secrets are configured
• Users will need to update their values.yaml to use the new secret reference format
• The new implementation is more aligned with Kubernetes best practices for secret management

Related Issues

• Fixes unleash#9111