Add ManagedCertificates and Backend Config objects

[ademariag]

Adds missing objects ManagedCertificate and BackendConfig, scoped to gke:: namespace

usage:

ManagedCertificate

 my_cert: kube.gke.ManagedCertificate("my-cert") {
    spec+: {
       domains: ["foo.example.com"],
    },
  }

BackendConfig

 my_cert: kube.gke.BackendConfig("config") {
    spec+: {
      cdn: {
        enabled: true
      },
    },
  }

[jjo]

@ademariag : btw you can run make -C tests local-tests to get a quick testing result from your changes.

[ademariag]

Thank you @jjo it was a disaster of a PR: thank you for your patience! Will fix and follow your suggestions.

[jjo]

Thanks @ademariag, after giving some thoughts, I'd prefer to keep this non-std kube additions in a separate kube-platforms.libsonnet file, created https://github.com/ademariag/kube-libsonnet/pull/1 over your personal branch, please take a look.

[ademariag]

Great, thank you. that works for me!