Add generic templates/networkpolicy.yaml

From gitlab-com/gl-infra/readiness!146 (comment 1297775574) and gitlab-com/gl-infra/readiness!146 (comment 1314653358) we want to add a NetworkPolicy resource to allowlist only the traffic we expect to reach our indexserver and webserver pods.

This is following the same pattern used in many of the resources in the main gitlab chart where we have a generic NetworkPolicy which basically gets all it's config from values.yaml and is disabled by default.

You can see how this will be configured in Staging by looking at gitlab-com/gl-infra/k8s-workloads/gitlab-com!2618 (merged)