Add documented examples for NetworkPolicies

[coro]

This closes #835

Note to reviewers: remember to look at the commits in this PR and consider if they can be squashed

Summary Of Changes

NetworkPolicies allow users to restrict the source/destination of ingress/egress traffic of their Pods. This documented example demonstrates the kinds of traffic a user may wish to restrict/allow on their cluster.

Additional Context

You can enable the network plugin on GKE for testing with:

gcloud container clusters create CLUSTER_NAME --enable-network-policy

Local Testing

Please ensure you run the unit, integration and system tests before approving the PR.

To run the unit and integration tests:

$ make unit-tests integration-tests

You will need to target a k8s cluster and have the operator deployed for running the system tests.

For example, for a Kubernetes context named dev-bunny:

$ kubectx dev-bunny
$ make destroy deploy-dev
# wait for operator to be deployed
$ make system-tests

[ChunyiLyu]

Looks great! Please see my comments for minor suggestions.

Plus, what do you think about adding a section in our rabbitmq.com doc to link to this example? I think rabbitmq.com could be a natural place for people to discover how to configure network policies.