Handle image pull secrets

[alexashley]

Updates the enforcer to grab any image pull secrets and send them when the auth key in the secret matches the registry.

I've been testing with the demo by leaving the Harbor project set to private and doing the following:

kubectl create ns test
kubectl label ns test enforcer-k8s=enabled
kubectl -n test create secret docker-registry harbor --docker-server=https://harbor.localhost/rode-demo --docker-username=admin --docker-password=$HARBOR_PASSWORD
kubectl apply -f pod.yaml

where pod.yaml is

apiVersion: v1
kind: Pod
metadata:
  labels:
    run: nginx
  name: nginx
  namespace: test
spec:
  containers:
    - image: harbor.localhost/rode-demo/nginx:latest
      name: nginx
      resources: {}
  imagePullSecrets:
    - name: "harbor"

[codecov-commenter]

Codecov Report

Merging #10 (2536c9a) into main (f65d011) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #10   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           97       133   +36     
=========================================
+ Hits            97       133   +36     

Impacted Files	Coverage Δ
enforcer/enforcer.go	100.00% <100.00%> (ø)

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