Add support for Kustomize

[sherifkayad]

Currently the only possible "official" way to deploy Gangway is to copy the yaml manifests under /docs/yaml and change manually in these and then applying to my cluster.

I would really love to have a Kustomization.yml added so that people who are using Kustomize can easily use this repo as a base and "customize" what they need.

This implies adding a Kustomization.yml file to the yml directory. Happy to do that myself if needed