[Feature]: Helm Chart Existing Secrets flexibility

Feature request

For the dashboard deployment, currently the OIDC and GCP exisiting secret logic is hard coded to do use the envFrom field, meaning all values must have the correct keys, and all keys will be loaded as environment values. This presents a challenge if you want to:

• Name your secret keys with different key names
• Include extra keys in your secret that you do not want to load as Environment variables
• Want to only include some of the values from a secret (for examble just the odc-client-secret), whilst specifying the remainder as normal environment variables (eg the oidc-issuer).

The logic also does not permit additional secrets or values from secrets to be added to the dashboard. environment variables can only be defined in the .dashboard.env field as strict key value pairs, with no option to specify them from a secret. There's also no way to include additional secrets or configmaps in the envfrom field.

Proposed solution

This request is to allow the fields to be more flexibly defined. Environment variables should be optionally a value from the helm chart, or an existingSecret with an optional key. Users should also be able to provide additional environment variables, and these should also be allowed to come from secrets.

This enables users who have a secret which only contain some of the OIDC or GCP values to use them alongside the static values in the helm values.yaml, and for the keys to be parameterised. This is a fairly common approach with helm charts that allows for better integration with other operators and k8s environments.

Additional context

No response