Enable alpha plugins by default in ArgoCD

Big Bang uses a custom image with plugins for ArgoCD. The SopsSecretGenerator plugin fails to load in the default configuration. You get the error loading generator plugins: external plugins disabled; unable to load external plugin "SopsSecretGenerator". In order to make the plugin work properly, we need to enable alpha plugins using the following:

addons:
  argocd:
    values:
      server:
        config:
          kustomize.buildOptions: "--enable-alpha-plugins"

We should have this set by default in chart/templates/argocd/values.yaml. Make sure there is a comment in there about enabling this for plugins.

Thanks to @matthew.kaiser for identifying the problem and solution.