Allow kustomize image override in Kustomization CRD

[somtochiama]

In a case where the person with control over the syncing(kustomization CRD) does not have control over the config repository where the kustomization.yml reside and they can only refer to it. It would be great want to allow making changes(overriding the images) in the Kustomization CRD

Link to discussion:
(https://github.com/fluxcd/flux2/discussions/135)[https://github.com/fluxcd/flux2/discussions/135]
In particular this comment

In the first, the person with control over the syncing does not have control over the config repository -- they can only refer to it. So they must resort to putting changes into the sync.

[stefanprodan]

Here is an example that could be used in e2e testing.

Having a public repo and a kustomization for podinfo:

apiVersion: source.toolkit.fluxcd.io/v1beta1
kind: GitRepository
metadata:
  name: podinfo
  namespace: default
spec:
  interval: 5m
  url: https://github.com/stefanprodan/podinfo
  ref:
    branch: master
---
apiVersion: kustomize.toolkit.fluxcd.io/v1beta1
kind: Kustomization
metadata:
  name: podinfo
  namespace: default
spec:
  interval: 5m
  path: "./kustomize"
  prune: true
  sourceRef:
    kind: GitRepository
    name: podinfo
  validation: client

I want to change the podinfo image tag without modifying podinfo repo, to do that I can override the image with:

apiVersion: kustomize.toolkit.fluxcd.io/v1beta1
kind: Kustomization
metadata:
  name: podinfo
  namespace: default
spec:
  interval: 5m
  path: "./kustomize"
  prune: true
  sourceRef:
    kind: GitRepository
    name: podinfo
  validation: client
  images:
    - name: ghcr.io/stefanprodan/podinfo
      newName: ghcr.io/stefanprodan/podinfo
      newTag: 5.0.0

[stefanprodan]

Implementation tasks:

• add the Image struct and spec.images to Kustomization API (docs+api pkg)
• implement the images override in the controller reconciliation
• add e2e tests for the images