[kustomize] Add a dry-run option to SubstituteVariables

[souleb]

We would like to add a dry-run parameter to the following func in pkg/kustomize:

func SubstituteVariables(
	ctx context.Context,
	kubeClient client.Client,
	kustomization unstructured.Unstructured,
	res *resource.Resource) (*resource.Resource, error){...}

It would become

func SubstituteVariables(
	ctx context.Context,
	kubeClient client.Client,
	kustomization unstructured.Unstructured,
	res *resource.Resource,
         dry-run bool) (*resource.Resource, error){...}

When the dry-run parameter resolve to true, we would replace the kubeAPI querys to retrieve substitutable objects by mocks.

We could then use it to enabled completely disconnected consumers (e.g flux cli in constrained environments) to use this function.