Implement var substitution from ConfigMaps and Secrets

[stefanprodan]

This PR adds an array field called substituteFrom to the post build API. SubstituteFrom holds references to ConfigMaps and Secrets containing the variables (data keys) and their values (data values) to be substituted in the YAML manifests.

You can disable the variable substitution for certain resources by either labeling or annotating them with:

kustomize.toolkit.fluxcd.io/substitute: disabled

Example:

apiVersion: kustomize.toolkit.fluxcd.io/v1beta1
kind: Kustomization
metadata:
  name: apps
spec:
  interval: 5m
  path: "./apps/"
  postBuild:
    substitute:
      cluster_env: "prod"
      cluster_region: "eu-central-1"
    substituteFrom:
      - kind: ConfigMap
        name: cluster-vars
      - kind: Secret
        name: cluster-secret-vars

Fix: #274
Followup: #253