feat: add overlay for a microk8s based single-user install

[ca-scribner]

This adds an overlay that configures Kubeflow's dex-auth and oidc-gateway charms with default settings applicable to a single-user install using microk8s based our tutorial. The purpose of this overlay is to provide a simplified first time install experience without damaging the flexibility of the general Kubeflow bundle. Without this overlay, our guides tell users to do:

juju deploy kubeflow --trust
juju config dex-auth public-url=http://10.64.140.43.nip.io
juju config oidc-gatekeeper public-url=http://10.64.140.43.nip.io
juju config dex-auth static-username=kubeflow
juju config dex-auth static-password=kubeflow

whereas with this overlay, we can simplify this to:

wget https://raw.githubusercontent.com/canonical/bundle-kubeflow/main/overlays/microk8s-single-user-overlay.yaml
juju deploy kubeflow --trust --overlay microk8s-single-user-overlay.yaml

[ca-scribner]

Left in draft until we decide this is the approach we want to take.