Add support to limit applied policies in automation by specifying a selector

[Nitive]

Hello! I want to automate external image updates to my repository, each image to its separate branch — something like Dependabot does. But since the ImageUpdateAutomation resource doesn't have a direct relation to ImagePolicy resources except for being in the same namespace, I would have to create a separate namespace for each pair of ImagePolicy and ImageUpdateAutomation otherwise updates would be merged in 1 branch.

The problem was previously discussed in #499 and fluxcd/flux2#107

The solution to the problem would be to have some kind of relation between ImageUpdateAutomation and ImagePolicy. I see 2 ways how ImageUpdateAutomation resource can be improved to support such a use case:

1. Have a list of policies and reference them explicitly

apiVersion: image.fluxcd.io/v1alpha1
kind: ImageUpdateAutomation
metadata:
  name: update-teleport
spec:
  policies:
    - name: teleport
    - name: teleport-helper
  ...

2. Have a policy selector and filter policies by it

apiVersion: image.fluxcd.io/v1alpha1
kind: ImageUpdateAutomation
metadata:
  name: update-teleport
spec:
  policySelector:
    matchLabels:
      app.kubernetes.io/component: teleport
      app.kubernetes.io/instance: teleport
  ...

In this PR I implemented the second option because it's more flexible, covers more use cases (you can batch updates however you like), and you can also imitate the first option with kubernetes.io/metadata.name label