cert-manager: Add cert-exporter CronJob

The `cert-exporter` tool fetches certificates from Kubernetes Secret
resources and commits them to a Git repository.  This allows
certificates managed by *cert-manager* to be used outside the Kubernetes
cluster, e.g. for services running on other virtual machines.

cert-manager/kustomization.yaml

@@ -5,6 +5,7 @@ resources:
 - cert-manager.yaml
 - cluster-issuer.yaml
 - certificates.yaml
+- cert-exporter.yaml
 
 secretGenerator:
 - name: cert-manager-tsig
@@ -20,3 +21,10 @@ secretGenerator:
   - zerossl.secret
   options:
     disableNameSuffixHash: true
+
+- name: cert-exporter-sshkey
+  namespace: cert-manager
+  files:
+  - cert-exporter.pem
+  options:
+    disableNameSuffixHash: true