Take action for omitted fields needed for external name before the generation starts

### What problem are you facing?

ExternalName configuration has a field called `OmittedFields` that we use to remove the fields that are filled by external name, such as `name` or `cluster_identifier`. However, the deletion of those fields from schema happens in CRD generator so other tools who do look-ups in the schema may still see them. For example, example generator has to iterate over `r.ExternalName.OmittedFields` as well as schema to accurately check whether the field is still there.

### How could Terrajet help solve your problem?

I haven't dug into it but we could somehow shift that deletion operation left and keep the schema as the single source of truth regarding what fields exist in the final object.