allow usage of category key

[daern91]

Support reference number

https://jira.commercetools.com/browse/SUPPORT-5264

Description

Currently is possible to use: externalId, namedPath and name (in that order).

Extending this feature also by key would allow better usage of the tool and > also make it consistent with the options in the export (IMPEX UI) where it is possible to choose the following options for categories: name, key, externalId, namedPath