feat: 🎸 support CronJobs

[oeduardoal]

Solve: #15

This PR allows k8s-digester to work with CronJob in Kubernetes

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[halvards]

Thanks for the contribution @oeduardoal, this looks great!

You'll need to sign the CLA before I can merge.

[oeduardoal]

Thanks for the contribution @oeduardoal, this looks great!

You'll need to sign the CLA before I can merge.

Thank you @halvards! I just needed to fix the code to pass on tests.

[halvards]

Thanks @oeduardoal! Could you please add a unit test for CronJob resources to resolve_test.go?

You can see how to programmatically create K8s resources in resolve_stub_test.go. It already has code for Pod and Deployment resources.

[breunigs]

I believe this PR also needs to add code to pick up .spec.jobTemplate.spec.template.spec.imagePullSecrets (and …serviceAccountName?) in createK8schain.

[oeduardoal]

@halvards tests added!!

[halvards]

Thanks for adding the tests @oeduardoal!

@breunigs is right though, this change also needs to handle registry authentication by picking up imagePullSecrets and serviceAccountName from the jobTemplate spec in https://github.com/google/k8s-digester/blob/main/pkg/keychain/keychain.go#L70

[jcortejoso]

@oeduardoal Thanks for the PR. We are facing this limitation and we'd like to know if there is any update to add this feature.

Thanks again!

[halvards]

LGTM, thanks for your contribution - and your patience!