Implement WaitForTermination deletion policy

[stefanprodan]

The WaitForTermination deletion policy blocks and waits for the managed resources to be removed from etcd by the Kubernetes garbage collector. The wait time is determined by the .spec.timeout field. If a timeout occurs, the controller will log an error and will allow the Kustomization to be deleted while the managed resources will be left in the cluster.

Example:

apiVersion: kustomize.toolkit.fluxcd.io/v1
kind: Kustomization
metadata:
  name: podinfo
  namespace: default
spec:
  interval: 30m
  retryInterval: 5m
  targetNamespace: default
  sourceRef:
    kind: GitRepository
    name: podinfo
  path: "./kustomize"
  deletionPolicy: WaitForTermination
  prune: true
  timeout: 2m

Fix: #711

[matheuscscp]

LGTM