Add support for existing K8s secret for database credentials

[lbanita]

Briefly describe the feature

• Reference an existing for Kubernetes secret to read the credentials for the database, instead of providing them inline in the valus file.

What problem does this feature solve? Please link any relevant documentation or Issues

• It allows deployment of Paralus Helm Chart via GitOps (e.g. ArgoCD), by securing the secrets in an external source and retrieving them via External Secrets Operator or other similar mechanism

(optional) What is your current workaround?

• N/A