Feature: Allow rules to be applied on a per-namespace basis

[bbetter173]

This PR adds support for the following config entries:

port: 9443
certDir: "./hack/certs"
healthAddr: ":8080"
metricsAddr: ":8081"
rules:
  - name: "docker.io rewrite rule"
    matches:
      - "^docker.io"
    excludes: []
    namespaceMatches:
      - "^kube-system$"
    namespaceExcludes: []
    replace: "harbor.example.com/dockerhub-proxy"
    checkUpstream: false

This allows different rules to be applied to different namespaces. In my personal use-case, this enables us to point kube-system and harbor-system to a global harbor instance that we run to decouple reliance on 3rd party repositories.

It also cherry picks the commits from this repository to inject imagePullSecrets into the pod spec: indeedeng#53