Add support to import Types, Custom fields

As titled. We want to import new types for the product assets. In the current state the cli doesn't support it.

Start by inspecting the CLI's current product-asset import entry point and how it represents existing types and fields. Clarify which new asset types and custom fields are required, then define completion as successful import of those cases with coverage for the supported inputs.

Written by the indexing model from the issue text.