Implement Kapitan plugin as ConfigMamangementPlugin sidecar

[simu]

See also https://argo-cd.readthedocs.io/en/stable/operator-manual/config-management-plugins/#sidecar-plugin

With the sidecar plugin architecture, we can use the upstream Kapitan docker image, since we don't need to be able to mount a single kapitan binary into the argocd-repo-server container to implement the plugin.

Follow-up to #90

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