Add option to automatically scale to 0 when there's no web traffic

This requires the KEDA HTTP Autoscaler extension to be installed. The readme and values.yaml have been updated accordingly.

Other notable changes:

• the egress rules for the evaluator and its test have been deleted. This is because the HTTP autoscaler required all the containers to be in one pod and network rules are specified at the pod level. We expect this to differ in production where each evaluator is likely to be in its own pod
• installing AppArmor policy has been documented
• the AppArmor tests are not run when AppArmor is disabled

Bug: T334247