Get external Policies `Constraint` and `ConstraintTemplate` resources

[mathieu-benoit]

In GitHub actions (in template repo), add a way to grab additional Constraint and ConstraintTemplate resources from an external repo.

Take inspiration of this like illustrated here:

- id: 'Download policies'
  # This step fetches the policies from the Anthos Config Management repository
  # and consolidates every resource in a single file.
  name: 'gcr.io/kpt-dev/kpt'
  entrypoint: '/bin/sh'
  args: ['-c', 'kpt pkg get https://github.com/GoogleCloudPlatform/anthos-config-management-samples.git/ci-app/acm-repo/cluster@1.0.0 constraints
                  && kpt fn source constraints/ hydrated-manifests/ > hydrated-manifests/kpt-manifests.yaml']