Allow configuration using env in helm chart

[nenadalm]

dex-k8s-authenticator allows usage of env variables in configuration: https://github.com/mintel/dex-k8s-authenticator/blob/98a3d45cb878f0baad9b3e19019fcbf6f9d42584/docs/config.md#environment-variable-support

but there seems to be no way to pass environment variables into the pod.

this pr allows passing data from secret as environment variables same way dex chart does it:

• https://github.com/dexidp/helm-charts/blob/8d69f098b027da77ea3f0507c7ab923a23adf6c0/charts/dex/values.yaml#L64
• https://github.com/dexidp/helm-charts/blob/8d69f098b027da77ea3f0507c7ab923a23adf6c0/charts/dex/templates/deployment.yaml#L65

[nabadger]

Apologies for such a long delay - thanks for this.

I'll most likely follow up with supporting env too (same way dexidp does it).