Develop a Kubernetes Network Policy for Sourcegraph

Created by: daxmc99

Sourcegraph would benefit from a more fine-grained Network Policy.

We already have some network policies that customers can leverage https://docs.sourcegraph.com/admin/install/kubernetes/configure#using-networkpolicy but they are not deployed by default.

Furthermore, we should make our network policies more fine-grained so that services only have explicit access to the other services they need to talk to. This also enforces the separation of services.

We could leverage https://editor.cilium.io/ to create this.