Allow selecting DecoratorController objects by field selector

[grantr]

DecoratorController can already select objects by label selector and annotation selector. It would be convenient to select objects by field value as well.

An example is the schedulerName field allowing a Pod resource to specify an alternate scheduler for itself. Here's a DecoratorController that acts only on Pods specifying the foo scheduler:

apiVersion: metacontroller.k8s.io/v1alpha1
kind: DecoratorController
metadata:
  name: foo-pods
spec:
  resources:
  - apiVersion: v1
    resource: pods
    fieldSelector:
      matchExpressions:
      - {key: spec.schedulerName, operator: In, values: [foo]}

[enisoc]

Oooh I like that!