Add common labels and annotations patching capabilities

[stefanprodan]

This PR adds an optional field to the API called CommonMetadata for setting labels and/or annotations to all resources part of a Kustomization.

Example:

apiVersion: kustomize.toolkit.fluxcd.io/v1beta2
kind: Kustomization
metadata:
  name: my-app
  namespace: apps
spec:
  # ...omitted for brevity
  commonMetadata:
    labels:
      app.kubernetes.io/part-of: my-app
    annotations:
      a8r.io/owner: my-team

Note that any existing label or annotation will be overridden if its key matches a common one.

⚠️ The main difference to the Kustomize commonLabels and commonAnnotations, is that the controller sets the labels and annotations only to the top level metadata field, without patching the Kubernetes Deployment spec.template or the Service spec.selector.

Closes: #508

[haarchri]

@transacid check this feature