Configurable Category Code Field

Problem/Motivation

The TaxJar Category Code field is not currently configurable. It would be helpful to allow site administrators to define a default Category Code per Product Variation Type, set the field as required, allow modifiable field description, etc.

Proposed resolution

Change the TaxJar Category Code field defined in `commerce_taxjar_entity_base_field_info()` to a configurable field

Remaining tasks

1. Create New Configurable Field
2. Migrate data from Base Field to Configurable Field
3. Remove Base Field