Store config file and access rules in configmaps

[natalian98]

In order to test this PR:

1. Pack and deploy the charm:

   charmcraft pack
   juju deploy ./oathkeeper*.charm --resource oci-image=$(yq eval '.resources.oci-image.upstream-source' metadata.yaml)

2. Wait for the charm to get active, then get the configMap for oathkeeper config:

   kubectl get cm -n <namespace> oathkeeper-config -o yaml

   The access-rules configmap should be created, but without rules data.

3. Deploy a charm that is able to relate with oathkeeper via auth-proxy (for example, zinc-k8s from this branch):

   juju deploy ./zinc-k8s*.charm --resource zinc-image=ghcr.io/jnsgruk/zinc:0.4.9 --trust
   juju relate oathkeeper zinc-k8s

4. Check the access-rules configmap:

   kubectl get cm -n <namespace> access-rules -o yaml

   Verify that the access rules repositories got updated in the config file:

   juju ssh oathkeeper/0 "PYTHONPATH=agents/unit-oathkeeper-0/charm/venv/ python3 -c '
   from ops import pebble
   p = pebble.Client(\"/charm/containers/oathkeeper/pebble.socket\")
   f = p.pull(\"/etc/config/oathkeeper/oathkeeper.yaml\")
   print(f.read())
   '"

   or run juju run oathkeeper/0 list-rules. You can also deploy traefik and relate it to zinc to verify that the access rules get updated.

5. Run juju remove-relation oathkeeper zinc-k8s and check the access-rules configmap to see that the rules got removed.