Support for k8s/microk8s credentials for image registries

To use k8s and microk8s for GitHub private repositories, it is necessary to provide credentials for the [`ghcr.io` repository](https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry#authenticating-to-the-container-registry).

We are currently doing this in `operator-workflows` for:
- k8s: https://github.com/canonical/operator-workflows/blob/cc6be7e6ae364e738e6d46205f7df45774dfa767/.github/workflows/integration_test_run.yaml#L263
- microk8s: https://github.com/canonical/operator-workflows/blob/cc6be7e6ae364e738e6d46205f7df45774dfa767/.github/workflows/integration_test_run.yaml#L324

This feature will allow us to use `concierge` in our GitHub pipelines.

This requests is similar to https://github.com/canonical/concierge/issues/75. That issue is related to the mirror for `docker.io`. I do not think that approach works, as a bearer token is necessary for authentication following the authentication flow.