Add support for other values for ImageUpdateAutomation `spec.sourceRef` field

[applike-ss]

I would love to see other reference options for the ImageUpdateAutomation spec.sourceRef field.

I do see use for a ConfigMap reference that holds image values.

Imagine these resources to be valid:

apiVersion: v1
kind: ConfigMap
metadata:
  name: my-configmap # This resource would not be managed by flux, as flux iua would update it and a kustomization would revert that.
  namespace: default
  annotations:
    image.toolkit.fluxcd.io/image=imageField
    image.toolkit.fluxcd.io/image-tag=imageTag
    image.toolkit.fluxcd.io/image-repository=imageRepository
data:
  imageField: xyz
  imageTag: v0
  imageRepository: index.docker.io
---
apiVersion: image.toolkit.fluxcd.io/v1beta2
kind: ImageRepository
metadata:
  name: podinfo
  namespace: default
spec:
  image: ghcr.io/stefanprodan/podinfo
  interval: 5h
---
apiVersion: image.toolkit.fluxcd.io/v1beta2
kind: ImagePolicy
metadata:
  name: podinfo-policy
  namespace: default
spec:
  imageRepositoryRef:
    name: podinfo
  policy:
    semver:
      range: 5.0.x
---
apiVersion: image.toolkit.fluxcd.io/v1beta2
kind: ImageUpdateAutomation
metadata:
  name: podinfo-update
  namespace: default
spec:
  interval: 30m
  sourceRef:
    kind: ConfigMap
    name: podinfo

In this scenario we are creating a non-managed configmap and a set of flux resources updating it.

I could also imagine enhancing the reconcile annotation to support a case to define action that are disabled, e.g.:

kustomize.toolkit.fluxcd.io/reconcile: disabled=patch