Support the restoring of issuers from backup files

[charlieegan3]

This allows the loading of cert-manager issuers and cluster issuers from a jsctl backup.

For example, with a backup.yaml as follows, the output for the installation will be as seen below. Note that the warnings of other issuers are logged to stderr in the process. In this case, a GoogleCASIssuer.

 $ cat backup.yaml 
apiVersion: cert-manager.io/v1
kind: ClusterIssuer
metadata:
  creationTimestamp: null
  name: cm-cluster-issuer-sample
spec:
  acme:
    email: dummy-email@example.com
    preferredChain: ""
    privateKeySecretRef:
      name: example
    server: https://
---
apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  creationTimestamp: null
  name: cm-issuer-sample
  namespace: jetstack-secure
spec:
  ca:
    secretName: ca-key-pair
---
apiVersion: cas-issuer.jetstack.io/v1beta1
kind: GoogleCASIssuer
metadata:
  creationTimestamp: null
  name: googlecasissuer-sample
  namespace: jetstack-secure
spec:
  caPoolId: my-pool
  credentials:
    key: example
    name: googlesa
  location: us-east1
  project: example
---



$ go run main.go operator installations apply --stdout --experimental-issuers-backup-file=backup.yaml
The following issuers cannot be managed by the operator and must be restored manually: GoogleCASIssuer/googlecasissuer-sample
apiVersion: operator.jetstack.io/v1alpha1
kind: Installation
metadata:
  creationTimestamp: null
  name: installation
spec:
  approverPolicy: {}
  certManager:
    controller:
      replicas: 2
    webhook:
      replicas: 2
  issuers:
  - ca:
      secretName: ca-key-pair
    name: cm-issuer-sample
    namespace: jetstack-secure
  - acme:
      email: dummy-email@example.com
      preferredChain: ""
      privateKeySecretRef:
        name: example
      server: https://
    clusterScope: true
    name: cm-cluster-issuer-sample

[irbekrm]

Thank you for working on this Charlie!

I think the new flag looks good already for a first iteration. I've left a couple commands, most notably about distinguishing different issuer types that might not be backed up.
Additionally it'd be good if we printed out which issuers got restored (it kind of is obvious from Installation spec for us, but maybe not for someone less familiar with that CR).

We also don't parse any List types at all, but I think this is okay for now.

[irbekrm]

Thanks Charlie, I've ran the new command and it looks good to me 👍🏼

/lgtm

[charlieegan3]

Thanks for the feedback and for testing this out too.