Helm chart - Add possibility to load credentials from existing secret

[mueckinger]

Describe the solution you'd like

For improved security users should not be urged to have credentials in their overriding values.yaml.
Best practice and Proposal: Add possibility to load credentials from existingSecret

Variant

Web Application

Container Management (if applicable)

Kubernetes

Operating System (for VS Code Extension)

None

[mueckinger]

I will create a PR for this