Support Configuration from ENV and/or File

[Skiepp]

From a Kubernetes perspective, this plugin currently supports configuration from Traefik Middleware or a fixed configuration in Traefik Configmap. It would be useful to have at least ClientID and ClientSecret readable from an environment variable or a mounted file.

This way this plugin could be better integrated into a Kubernetes environment, especially when there are tools like Keycloak Realm Operator that configures Keycloak access with a Secret.

I'll work on a PR.