Feature Request: building only CRDs

[p3lim]

This relates to #1049, as it's useful for bootstrapping.

With flux-local build we can specify --skip-crds, but I'd like to see the opposite; --only-crds.

This isn't crucial, as one could just use yq 'select(.kind == "CustomResourceDefinition") to filter this from flux-local build all, but it would be nice not having to use a separate tool for this.

[allenporter]

I think using kustomize cfg grep on the output woudl be the way to go, assuming it can work.