Allow env variable string substitution in rewrite rules

[matin-nayob]

Firstly, thank you for this project, it really is awesome!

We have multiple isolated K8s clusters, with separate Harbor instances in each one, accessible on different domains.

It would be super neat if the rules config allowed you to substitute parts of the replace section with an environment variable value, like:

replace: '${HARBOR_DOMAIN}/dockerhub-proxy

As we use kustomize with helm to generate the final k8s manifests for this project, we end up repeating the values.yaml file for each cluster with only the harbor domain being different in each file.

Not a big deal, but if you think its a good ideal, thats cool.

Once again, thank you for this project, it really has made switching to Harbor so much easier.