Add NetworkPolicies to isolate ArgoCD from rest of cluster

[HappyTetrahedron]

Implements #149

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[HappyTetrahedron]

I figured that for non-openshift, we can specify the monitoring NS via allow_from_namespaces. I didn't feel like cross-pulling that info from all over the hierarchy.

RKE is as yet untested though; gonna do that on Friday.

[simu]

For setups which use component-prometheus, we should be able to generate the correct NetworkPolicy for the monitoring stack via component-prometheus's component library with prometheus.NetworkPolicy(), see also the example in https://hub.syn.tools/prometheus/how-tos/cluster-monitoring.html#_advertise_metrics_from_a_component

[simu]

I figured that for non-openshift, we can specify the monitoring NS via allow_from_namespaces. I didn't feel like cross-pulling that info from all over the hierarchy.

As mentioned in the previous review comment already, we should use prometheus.NetworkPolicy() which exists exactly so that components can provide seamless integration with component-prometheus instead of requiring users of component-prometheus to explicitly configure their Prometheus namespace when network policies are enabled, especially since we already handle the other half of it by wrapping the namespace in prometheus.RegisterNamespace() when component-prometheus is installed on the cluster.

[HappyTetrahedron]

As mentioned in the previous review comment already, we should use prometheus.NetworkPolicy()

I should learn how to read 🙈

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[HappyTetrahedron]

naw I did it wrong