Add kustomize based extensions to the base manifests

[MoisesGSalas]

Description

This adds an additional manifest builder that creates an additional overlay on top of the ones generated by BaseManifests. ExtendedManifests will take the path defined in extra_templates in your drydock reference and render the templates living there on to your drydock-env.

How to use

1. Modify you reference to include:

  manifest_class: drydock.manifest_builder.infrastructure.tutor_based_manifests.ExtendedManifest
  manifest_options:
    extra_templates: extra_templates_root

2. Create the directories extra_templates_root/extra-extensions and add a kustomization.yml with you additional resources. Remember that this is a template and you have access to all the variables (e.g. {{ K8S_NAMESPACE }}).
3. Run tutor drydock save -r reference.yml to generate the new environment.

Ditto of #2 on the correct branch