Add a kustomize file to install the four manifests.

[raykrueger]

What would you like to be added:
I would like to add a kustomize file so this repo can be installed with kubectl apply -k vs using the clone and make approach, or doing kubectl apply -f with four different links.

Why is this needed:
Currently users are cloning this repo rand running make cluster-up which is extra steps. Alternatively, we could just add four kubectl apply -f lines to the repo README that installs the manifests from the [Makefile]j(https://github.com/aws/amazon-eks-pod-identity-webhook/blob/master/Makefile#L76).

Personally, I think the one line kubectl apply -k is nicer.
I'm happy to submit a PR for this.